The problem, in my eyes, is that they keep making them Mario-like, despite the 2D-3D switchover making that absurd.

2D platformers were straightforward mechanically, with dead-simple controls and a clear field of view. Mario spiced the format up by adding lots of novelty mechanics (play as Luigi, get a tanuki suit, throw enemies at each other, spit fire, swim, use a submarine) and rewarding exploration (full of hidden bonuses and secret worlds), while Sonic spiced it up by doubling the gameplay speed and making it about responding to onscreen events quickly and racing the timer (thinking underwater levels here).

Both worked really, really well.

But 3D platformers have been dominant for 20 years now, and they play differently. They're more complex mechanically, taking place in 3D space with a camera to keep track of, more dimensions to keep track of, and more complex controls as a requirement. You can't just make Super Mario Bros. 1 3D and expect it to be fun, so Nintendo didn't, they made their 3D platformers slower but more open and exploration-heavy, and it worked.

Sonic just slowed theirs down. Sonic 3D are rarely as fast as Genesis games; Sonic Adventure and Sonic 06 expect you to operate more slowly, to carefully prepare and time jumps, pivot the camera to see where to go next, to stop and find the 1-ups and animals hidden in crates and behind scenery. They add tons of new playable characters (Amy, Silver, Shadow, Omega) who move slowly. So do they go the route of Mario 64, and make larger, free-roaming worlds with goals you can pursue in random order? Nope, they're still linear levels with boost pads and loops everywhere. You're just way slower moving now and the game will punish you for actually going fast and using them. Any time the game actually does move quickly it's almost completely on-rails, you hardly have to do anything.

I think their fatal flaw was trying to be too much like 3D Mario, but not having the guts to be completely like 3D Mario: they keep trying to force the trademark speed mechanic into games where it just doesn't work. The fact that booster pads litter the ground is an indicator of this: the Genesis games expected you to be moving quickly most of the time, and loops would enforce this, while 3D games expect you to be moving slowly most of the time and helpfully put you in fast mod before any loop, making them pointless visual-only things.

They should have either gone all-in on the 3D platformer model, or turned Sonic into more of a racing/endless-runner hybrid without pretentions of platforming at all.