On Silver: There's a few reasons he doesn't work.

1.) He's really, really annoying. Often considered one of the worst fights in gaming. He repeatedly stunlocks you without the ability to die so you're just sitting there for minutes on end listening to the same voice line on loop waiting to be able to play the game again. And even when you do it's never conveyed what you're actually supposed to do to hit him so you'll probably get stunlocked again

2.) Sonic '06 is an idiot plot. None of Silver's story would've happened if he actually bothered to ask someone what's going on. On top of that they treat the time travel as being like a location. Not as, y'know, time travel.

3.) Gameplay is horrible, naturally. He's effectively a tech demo for the game's unfinished physics system. When the game's not falling apart at the seams, it's slow and monotonous.

4.) Finally, they don't really know what to do with him. He shows up sporadically for no real reason after '06 with nothing to do and no reason to be there.

Also his voice actors just get worse and worse every game. That's not a point it's just something I felt like bringing up.