You didn't really address the assist trophy argument, though. Assist trophies, much like DLC Mii costumes (at least for characters who aren't already playable)
have consistently been treated like consolation prizes since they were first introduced. (which, in the case of assist trophies, was over a decade ago - the picture's not from when assist trophies were introduced, but it helps to show that being an assist trophy means that a character won't be playable in the same game)
There's no guarantee that we'll even get an assist trophy promoted to playable through DLC, and the fact that we got Min Min over Spring Man (much like Pyra/Mythra over Rex) seems to be a sign that it might not be happening. (I'm sure the argument could be made that ARMS's producer said "everyone's a protagonist", but Spring Man being the face of ARMS, an assist trophy in Smash, and not even being considered for a playable role in Smash's DLC seems pretty damning from my perspective - from what Sakurai said, the choice was between Min Min and Ninjara)
I did address it many times,
-This Waluigi picture has multiple impetrations. You can read it as saying that Waluigi is unplayable because he's an Assist, You can read it as Waluigi being unplayable for specific reasons, and then being made an Assist because he was Unplayable. There's also the fact that Sakurai might have wanted to make sure people aren't confused that Waluigi is Unplayable. he did this with Skull Kid and Ashley as an example
This also doesn't take into account that DLC likely wasn't Planned for Smash 4 at this point. Just like the point about how Ultimate's DLC was said not to be Decided to be a thing till sometime after January of 2018, when the base game content was set in stone, might have been the same deal here
The thing to take into account is that the situation with Smash 4 and Ultimate is a different story. Smash 4 didn't have this much extension on it's DLC, since Ultimate has FP2.
I think the Important thing is that Sakurai never addressed the Assist Trophy thing, when he could have on Multiple occasions. He made sure to reiterate the no Manga character rule as an example
And I think each Assist trophy is in a different scenario. I think a Problem is that people look at Every Assist trophy and assume the situations are the same, when I think there could be reason to believe that's not the case. Regarding Spring Man, they might have already had the idea he wasn't gonna be chosen, so they may have made him the Assist cause they knew it was between Min Min and Ninjara. It also helps that Spring Man was newly introduced in this game, so it wouldn't have been weird if he wasn't an Assist
Shadow is a veteran Assist, so his absence would have been weird, and they probably negotiated for the Assist before DLC was decided to be a thing sometime around January of 2018. We also see that with the fact they even still had ARMS Content and Xenoblade Chronicles 2 content in base game that FP2 wasn't a sure thing that was gonna be developed.
I refute this because Shadow was a clear favorite all throughout base game speculation, many people pretty much considering him a lock as an echo fighter.
Perhaps you interpreted people’s expectations being regulated to echo fighter as an “agenda” against him, but you have to remember what the situation was at the time. There was no precedent for adding fully unique second “reps” from third parties with a character already in the game. We got Richter though, which got people thinking about other third parties - namely Ken and Shadow.
Like it or not, Shadow fits an echo role pretty well at face value. Maybe he could get a new Up B like Chrom and a new Final Smash, but otherwise Shadow is a literal clone of Sonic and is capable of pretty much everything he can do. I personally think Shadow ought to be a Luigi or Wolf esque derivative if anything, but you can’t blame people for trying to be realistic. It was really the only chance he had.
And then most people just take Assist Trophies at face value and believe those characters are out of the running until next time. You really can’t fault people for that, it’s very much an “I’ll believe it when I see it” deal where there’s zero precedent to expect AT promotions right now and no clear reason to pick Shadow specifically.
Richter was breaking Precedent by himself though, just being in the game. Shadow could have broken precedent being the 1st unique secondary 3rd party character in Base roster as well. I thought this was especially true with Isabelle showing that Semi-Clone type characters were still a thing.
But I do get what you mean. People just followed the Precedent set at the time, and to be fair this precedent did last up until Sephiroth in FP2, and I know many people weren't even looking at his Direction (I was though, but he was not active prediction when he got in)
But honestly, I could tell that even if it was proven that 3rd parties could get Unique characters, just based of the responses I've gotten even back then, people would still say that Shadow could only be an echo. even among people that might believe the Assist trophy thing is not an obstacle, that's still a saying that's ingrained in the brain
I do know that Shadow does work as an Echo, and if he really didn't have the merits to be his own fighter, than I would think the Echo Fighter was more likely. The thing is that he does have his own Merits to be his own Fighter, even over Eggman and Tails, but people choose to ignore it or not give it the time of day
But I don't have a problem with people that Expect Eggman or Tails (even Knuckles) over Shadow, since they all have their reasons to be in. I still think there's an Agenda though, because if you look at the responses after the Base game speculation ended, you see way too many times about how "Shadow was such an easy Echo to add in and Sega must have messed something up". People many times say Shadow was deconfirmed again when they said no Echo Fighters.
So when I see people, even people that do believe Assist Promotions are possible, not give Shadow the time of day when he's more recognizable than Sephiroth even, and he's been a Popular request for years and years, It does make me think there is an agenda, but I don't think there's an agenda going on if someone thinks Eggman and Tails would get in, because it's focused on Eggman and Tails merits, not downplaying Shadow's merits