I don't think Tails would have been a popular enough request to be considered by Sakurai. But really I'm not concerned too much. I think some people get the impression that Shadow getting in Smash over Tails would somehow establish him as a more important character, but I don't think that's the intent behind the characters added in Ultimate. Smash is simply giving the fans what they want, and Shadow is more requested by fans. Tails has already been cemented as Sonic's player 2, it's
his role. However Sakurai chooses to represent the Sonic franchise in Smash, it won't have any impact on future Sonic games. It's not like we're gonna see Shadow flying on the Tornado with Sonic in the next game.
Really the only bad thing I can say would come from it is the fact that I won't be able to play as a character I like, but that obviously applies to many of my other dream characters already. Unlike some of the other characters I like, Tails is lucky enough to still have developers doing him justice in the modern day, so I'm thankful for that.
I don't think anyone really doubted that Shadow has more popularity than Tails. But since Shadow is most likely going to only take up an echo fighter slot, it's whether or not Tails ALSO did well enough to the point where he'd be warranted as the 3rd Sonic character and the 2nd Unique Sonic character. I do not think Shadow having a near lock chance of being in the game hurt's Tails' chances at all. Sure, you can't have 2 Sonic echos, and you probably sure as heck aren't getting 2 unique Sonic characters, but having 1 new character and 1 echo is very much possible.
Obviously, taking the easier route with echo fighter Shadow only is something they could possibly do, but we don't know that yet as they have yet to show us Shadow as well. I wouldn't give up hope on the fox just yet.
As for Tails getting justice from the developers, that's only with Classic Tails in the form of Sonic Mania. They have not been kind to modern Tails. He's not a playable character in Unleashed, Colors, Generations, Lost World, AND Forces. Making his last 3D playable appearance like what.... Sonic 06?
They also managed to strip away any sort of courage he gained from Sonic Adventure 1 & 2 by making him the coward that he is in Forces.
We most certainly wont have to worry about Shadow taking Tails' "place", but Tails himself has really lost who he is in this last decade of gaming (again, outside of the mostly fan led Sonic Mania). Much like K.Rool, Tails could really use a big boost in popularity by becoming a playable fighter in Smash. It would also finally make up for the lack of a good playable Tails in well over a decade, something I'm sure most Tails fans have been longing for.
Lastly, I want it to be clear that while Tails hasn't had any traction lately, that doesn't mean he did bad on the ballot. He just has not appeared in any of those fake leaks or hardcore fan polls to rally up the meme band-wagoners like Waddle Dee, Skull Kid, Shantae, ect. Tails is a very casually well known character, unlike most of those listed just now. He very well could be the silent casual majority that simply voted in the ballot and did nothing else. You never know. At the very least, we have the Mii Costume theory to rely on, as they've already proved to have many connections with characters appearing in the games.