Think about every 3D Sonic game, consider their core gameplay(whether from mechanics or level design) flaws, inexcusable/inexplicable focus on superfluous shit like cutscenes, voice acting, and alternative half-baked gameplay mechanics/gimmicks.
Imagine it's 2007 and instead of releasing Sonic 06, SEGA made Super Mario Galaxy lock stock and key, but Mario was reskinned as Sonic and so were the enemies. The game would have been heralded as the best Sonic game ever made.
The 3D games have no gameplay identity or "feel" like the 2D Sonic games, which are momentum/pinball based 2D platformers with levels that compliment the mechanics by having multiple paths and few areas that really punish the player for falling.
3D Sonic games, on the other hand have a hard time being defined singularly. They have done almost everything, poorly at that. It has been a brawler, a linear go fast boost game, a game where you seek for items, etc.
So SEGA has nothing to lose by going back to the drawing board and just emulating an actually successful, good game like Mario Galaxy, but reskinning it with Sonic.
Make tweaks, adjust the speed or momentum Sonic carries. Just make a pleasantly controlling demo with level design that actually compliments player actions and make enough content based around that to fill a 20-40 hour game.
Trim all the fat, the story can be read in a fucking manual. Eggman is turning flickies into Badniks to further an endgame of his.
Sonic is the only playable character unless you make skins of other characters that just play basically like him.
Doing this takes time and someone with the same vision. SEGA only ever allots a year for development and much of that is spent on nonsensical garbage like cutscenes and shitty gameplay gimmicks.
A little post script, the last 3D Sonic game I played was Heroes, I don't feel I've missed much.