I don’t understand the hatred of Sonic’s moveset, it really fits him perfectly, since what else would Sonic do. Sonic was put into Smash in 2008, before the boost titles, so there really was little to pull from. Sonic uses spin dashes because that’s what Sonic’s known for, and really was the only thing he could do at least in the main line games. He took the homing attack from Adventure and the charged spin dash from the classic titles. He plays exactly like Sonic, which is what Sakurai strives for when making characters, or else, why wouldn’t he have move changes in later installments.
As for the ‘darker’ argument, I think Sega wants to forget about Shadow The Hedgehog and 06, or at least parody them and admit they messed up. The darker titles were probably the worst era of Sonic, so why should Sega keep him so dark and have Smash represent it. Shadow has really lightened up in recent times so having cheesy dialog makes sense.
You also bring up how Sonic’s moveset doesn’t benefit Shadow, but that’s far from the case. Shadow, more often than not, is portrayed as a Sonic clone in gameplay, as he really is a darker Sonic. He may use Chaos Powers in cutscenes, but it usually doesn’t come in to play when using the character. As for how Sonic’s play style can be changed to fit Shadow, I already made a post about it.
They could have changed some of his moveset in Smash 4 AND Ultimate. By that time Sonic has adapted the boost formula. Not to mention other attacks like the light speed dash, bounce attack, somersault plus slide,Ring toss, etc. having TWO Spindashes is just...ehh, especially since Sonic could do so much more. the only thing i could think of why Sonic is unchanged is because Sakurai doesn't like to do moveset overhauls alot of the time. It Took 4 Games in order for Ganondorf to use his Sword (barring Customs).
As for the Darker argument, i Don't mind the Cheesy Dialog for Shadow, but if you look at Shadow and look at Sonic, Sonic is a more light-hearted character compared to Shadow's more dark Heartedness. Just because we are not in an era where Sonic games are Dark, Doesn't mean Shadow has become light hearted, He is still Darker Character than Sonic even today
as for the Sonic clone aspect. if you look at all the games where Shadow plays similar to Sonic, those are games which doesn't really make it's characters distinct from one another. SA2 had Rouge play like Knuckles and Tails play like Eggman. Heroes there were a few differences but not to may distinct ones between the types of characters in that game. Rivals 1 and 2 had everyone as a Sonic Clone. Forces I'll give you credit, but considering that was ever the first Time Shadow was playable in the boost formula, along with the fact he wasn't in the main game but was Free DLC, well, yes. in games where the characters are Really Distinct, Shadow always had substantial Differences from Sonic, with his Chaos abilities and all that
even in games where Shadow plays similar to Sonic, He still uses his Chaos powers in Some Way. SA2 had the multiplayer which Had Shadow use Chaos Spear and Control, Heroes has Team Dark use Chaos Control, which freezes time for 5-10 seconds. Sonic Rivals, in which everyone was a Sonic clone, had Shadow use his Chaos Control as his ultimate attack. Forces is the only game where Shadow has ever not used his Chaos abilities, but even in that game had him use it in cutscenes
speaking of Cutscenes, Characters in Smash aren't always based on Gameplay aspects, but also Concepts. if we were to just base characters on Gameplay aspects, Rosalina would only have a Spin attack to her name for example. Hell a good amount of the characters in Smash wouldn't be fighting right now. Cutscenes do play a role for a Character, in in Cutscenes Shadow is alot of the time seen overpowering Characters with his Powerful hits and Chaos abilities rather than Sonic's more hit and run fighting style in Smash. Boss Fights also matter. In pretty much every Boss fight has Shadow use his abilities as well. Sonic Boom was the most recent game that had Shadow as a boss fight, and in that game he uses his boost, he uses His Chaos Spear, and his Chaos Blast.
another thing to prove is that the one time Shadow was in a fighting game which was a DAMN GBA Game (Sonic Battle) He played Completely different from Sonic in that game, mostly using his Chaos powers and also being more high damaging character. This hits home that in an actual fight, Shadow makes more uses of Powerful hits and his Chaos Abilities rather than Hit and Run with Spindashes like Sonic
Shadow can be considered an iconic character at this point. He's been in many games, and while he has times where he played similar to Sonic, He's also had games where He's played Substantially different.His Chaos abilities are his trademark and associated with him greatly. They even follow him outside of games and even into other media like Tv Shows (Sonic X and Sonic Boom). To have Shadow basically be a Echo Carbon Copy of Sonic with TWO SPindashes and a Spring with just Property changes n my opinion undermines Shadow.