Recognizability is a bit of a double edged sword in Shadow's case. Sure, it's arguable that on the internet the rival of Sonic that debuted in Sonic Adventure 2 and made mostly regular appearances since then is going to get most who watch the reveals say some variance of "Oh it's Shadow!" maybe even in some cases more then the major villain from Final Fantasy 7 from just looks alone. Though perhaps that's why the moment most knew the character was Sephiroth was not when we got a close-up of Sephiroth... it only took just a few notes of One Winged Angel to make that happen. Shadow has themes of his own, but he hasn't had a consistent theme through all his appearences.
For fans like us, it might be easy to have a musical indication of Shadow since we may know at least the opening or beat to Throw It All Away (SA2), This Machine (Heroes), All Hail Shadow (Shadow's game/Sonic 06), or All I Am (Shadow's game) if for some reason they did a trailer for Shadow in a similar fashion to Sephiroth. But some of Shadow's recognizability comes from... the rather negative side of recognizability in that many of his post-SA2 appearances ruined his character for a good chunk or just makes him a laughable meme for those outside the Sonic fanbase. Granted the Sonic franchise itself has been kind of a joke on the internet ever since Sonic 06. But Shadow the Hedgehog is a particularly rather ridiculed part of Modern Sonic, certainly mostly stemming from his own video game where Sega made the weird decision to have him use guns and even... swear. (Granted it's only the words Damn and Hell) A wild shift from the generally family-friendly toned Sonic franchise. Sure this sort of publicity arguably makes him more in the spotlight then Sephiroth. But it's not exactly the good kind of spotlight. Sephiroth's kept his reputation over this years as this badass villain (Other then some inter-final fantasy fandom wars where they may call him a Momma's boy when comparing him to say... Kefka). While Shadow has some of the stench of the Sonic franchise's downfall from grace around him.
While technically the entire franchise may be dragged down by this logic, there's a keen sense that the smarter decision should Sonic gain a non-echo character is add a more classic character that still has some ties to the franchise's golden age (Which is surely when Sonic 3 was out. I think the early 2000's was a big step for Sonic as well and even when I first became a fan. But there should be hardly a doubt that the mid-90's is a place the Sonic franchise will never get to have back again. Similiar to how aside from a brief moment of Pokemon Go. Pokemon still hasn't been quite the cultural phenomenon it was in Gen 1/Early Gen 2 (I only mean of course in terms of how much attention the franchise was getting, I'm not implying there hasn't been good Pokemon games since then. I'm not an ignorant Genwunner. Though it arguably has applied to Smash because if you count Pokemon Trainer as 3 characters. 6 of the Pokemon are from Gen 1. 7 if you decide to include Pichu among Generation 2 where perhaps the popularity was still at a pretty high peak once Gen 2 started)
I get that the fanbase for Shadow can be very loud and vocal. But characters like Tails, Eggman, and Knuckles arguably have more of the recognizability factor among those who are only a little vaguely familiar with Sonic from his classic days who aren't going to show up in Favorite Sonic character polls. And while I know you continue to refute it, for how ever long Echo Fighters are a thing in Smash. Shadow's among those characters that make the most sense. I also think recent matters may put a dent in Shadow's favor in terms of 2 things: It's Mania, the classic game with Sonic, Tails, and Knuckles all playable that's had the largest success in the Sonic franchise in a long while. While in Forces the game where only Sonic, Classic Sonic, an Avatar, and Shadow in a few bonus levels (And any modern Sonic level after you've completed the Shadow bonus levels) did not do so hot. Which may just be unfortunate circumstances in terms of the most recent mainline games. But there's also now the live action movie(/s) that had only Sonic and Eggman in the first. And Tails front and center in not just a post-credits scene, but in a coming sequel. And the first Sonic movie turned out to have one of the best box office performances for a video game movie ever. Perhaps it's just two unfair coincidences that gives more credence to more classic characters. But it's why Shadow's supposed popularity in the Sonic fanbase may need a little more strength backing compared to what is doing the best financially and critically for the Sonic franchise at the moment. And neither Mania or the live action movies have much of a sign of Shadow in them. It might take something like an Adventure 2 remake to put Shadow back fully in that sort of fawning spotlight. (Though I'd certainly welcome an Adventure 2 remake even at the cost of potentially making Shadow a bigger threat to get in Smash before Tails. Just because I'd fanboy so hard at such an announcement lol)
-I know Shadow has alot of haters and ridicule. I don't think that the amount of hate Shadow gets will overshadow the amount of love that Shadow gets though.
Plus one could say this could give publicity in a way. Shadow's diviseness could actually be a way of giving more publicity to Smash. I know not all publictity is good, but you can't lie, that there may be some interest in asking the question of how Shadow got in over Eggman, Tails, & Knuckles, how Smash might be celebrating a Legacy that shouldn't be celebrated by adding Shadow, all of that. honestly this is a stretch. don't read too much into this excerpt
-Don't forget that SA1 and SA2 are considered Gems in the franchise as well. I understand the Classic era was a bigger deal, and the Adventure games have gotten flak in the 2010's, but remember that the Adventure era also did get in alot of New Sonic fans, especially when SA2 hit the Gamecube, as that brought in alot of Nintendo fans and such.
You also see alot of people wishing to return to Adventure era gameplay, and Many people want a Sonic Adventure 3.
-Eggman, Tails, & Knuckles are more Recognizable than Shadow, I'm not even gonna refute that
What I will say is that, to be honest, I don't think we are at a point where the amount of Recognizability gives those 3 that big of an edge over Shadow. Shadow himself is still a Recognizable character anyway, and even if people might not know exactly about Shadow, they still will at least know he's Sonic related, and might even be interested in this "Evil or Dark Sonic" if this were there first introduction into him.
If we were talking about the Super Casuals on the street, than yes maybe some won't even Recognize Shadow like that, or they'll just Recognize Shadow as Evil Sonic, but they more often will recognize Eggman, Tails, and Knuckles. at the same token, these same people will Call Steve "Minecraft guy", refer to Link as Zelda, and might not even recognize Sephiroth like that either
Anyone that's Casually into games, and would be ones that Nintendo is targeting with the Fighter Pass, will Recognize Shadow. That's the gist of it
-Forces Sold strongly though, as Confirmed by Sega. No I don't think it did better than Mania,, but unlike it's critical score, Sonic Forces was a success and that can't be denied. And I don't believe they factored in the Mobile game, which also has been very successful
-Regarding the movie, I can't really talk now. I will say that Really, Eggman mainly benefits from the movie, not Tails, since Eggman had a big part of the movie. Sonic Movie 2 hasn't filmed, so if someone introduced to Sonic through the movie went into looking to buy Fighter Pass 2, they aren't gonna be too familiar with Tails. He only appeared for like a minute, or less. They'll know Tails as that Fox from end of the Movie, not as Tails
Doesn't mean they wouldn't have interest, since I Imagine that them not knowing the role of Tails will entice them to see what the character is about, if they were in FP2. On that Same logic though, this could be applied to Shadow & Knuckles, despite them both not being in the movie. Shadow is gonna be Dark Sonic to these people, so they'll wonder why Sonic is so Dark and "Evil" and Edgy, and with Knuckles, because they know about the echidna Clan, and Knuckles is an echidna, that also sparks some interest
-Regarding the Echo thing, I'll admit that yes Shadow does work as an echo, and honestly if Shadow didn't have the merits to get in as his own character, than I would throw in the Towel
The thing is that that's not the case. Shadow does have his own merits to get in as Playable. you might not think that because you don't think Shadow can get in over Eggman and Tails. I understand that sentiment, and I'm not mad at you if you believe Eggman and Tails would get in over Shadow. They have their merits deinfitly to do so
I'm just saying that I do believe Shadow does have the merits to get in over the other characters. I do understand that yeah, It is kind of hard to see the reasons why, especially since Eggman and Tails could cover these options. But I wouldn't count Shadow out since yes, His popularity is still a big thing. Sega still gives him alot of presence, and His Legacy, while heavily flawed, did get alot of new people into Sonic.