I never got into Sonic, too simplistic. The Adventure games were a bit better and Sonic 06 would have been fine if it was actually finished, but the 2D stuff is just "hold right, press jump occasionally". No wonder Mario won that war.
I like both Mario and Sonic, but if I had to choose, it would definitely be Sonic. Sonic 3&K is basically the epitome of greatness, easily my favorite game of all time. I've always been enchanted by the silent cutscene style of the game, that conveys so much with nothing more than sprites and background details.
As far as your issue with the gameplay goes, I can't say I agree. Saying that it's just "Hold right to win" is a shallow assessment, since the games were originally designed with speedrunning in mind, long before the term was even coined, so the true way to play them is to play them again, but do it
better. More rings, more score, faster time, never getting hit, etc. Over time, you'll develop a sense of rhythm with the stage, blowing through them like the high-speed rollercoaster they were meant to be. Flying through the stages with expertly timed button presses feels
so good.
The
real problem is that getting to that level takes time. Back then, the only games we had were what our parents would get for us, and as long as chores and homework were done, we had a lot of free-time to kill. As gamers got older, we've found ourselves with less time to spend with games for various reasons, and a much wider selection at that. As such, the culture of playing games over and over to memorize them has faded away outside of dedicated speedrun circles. Now, single-player games are usually a one-and-done type of deal for most people. You beat the game, enjoyed the story, so now it's time to shelve it for something else.
So under
that particular lens, yes, the 2D games feel hollow, because the generous ring system allows you to tank damage almost indefinitely, so there's almost no challenge until the final levels where there's no rings and you're forced to do a no-hit run. Playing it only once doesn't allow you to achieve that perfect run, since you've never seen the stage before, making crashes, deaths, and other mistakes far more likely to occur, which I imagine for a lot of people who just "don't get" 2D Sonic find rather disappointing. I think they want to fly through the stages like I do, but since they didn't grow up with the games, they lack both the game knowledge and the context that the games were released in to find the appeal, which is the replaying the game, getting better and better each and every time.
TL;DR, you have to
EARN the speed in those games, and a lot of people don't get that.
I really do dig Shadow The Hedgehog and would love to see a game like that return. The game as is, right now, is pretty janky and suffers from "complete the game bajillion times to get the best ending" syndrome but it's got good foundations: Actual characters that are allowed to be evil and/or edgy, branching story with multiple endings, several objectives to be done on every map and good combo of platforming and combat. It's a shame it got shat on by the spergy sonic autists so it will never likely get a sequel
Oh, and music is top notch too, but Sonic has been generally good with music even in it's poorer entries.
I've had something of a love-hate relationship with
Shadow. At first, I thought it was good, then I thought it was bad. Now, I find it charming in that "So-Bad-It's-Good" way. The story makes no god-damned sense. The game's trying to be what it thinks being "mature" is, which amounts to dark metal, guns, swearing, and riding motorcycles. Hell, one of the endings is Shadow becoming so emo that he decides to hang himself, which is
so 2000's edge.
But peeling those issues away hides a game that's actually pretty good. If you ignore the "edginess" that just comes across as goofy, the game is actually a pretty solid run-and-gun platformer, and as you said, the music is really good.
