This quote from the Jim Sterling review I agree with the most:
They found the perfect Sonic formula in the 3D sections of Generations, so it's no surprise to me that when they change it, it makes for an inferior game.
I would normally agree with the idea that "if it ain't broke don't fix it", but Sonic is a bit of a special case, and the Unleashed game play in Generations was far from perfect. The issue with that is it turns what should be a gimmick into the core mechanic.
A platformer is only as good as it's design allows it to be, but that design is always built around the core mechanics. If you look at Mario, the reason he's been so consistent while still evolving with each and every installment is that at the series' core, there are only really two important things a Mario game needs to get down - running and jumping.
The Adventure formula actually worked decently in Sonic's jump to 3D but it was rather evident when people favored the Rush series that they wanted to try to have a go in translating that fast game play to 3D. But Sonic games were never about speed exclusively, they had good design built around them. The best kind of design you can get out of a game as fast as Unleashed is linear corridors with little to no control around them because the speed strips you from the control and you're not able to see many obstacles. Where do you go from there if you're to add things above that? How do you evolve a game which is based on a mechanic where the sole purpose is to go as fast as possible?
It's why I think slowing Sonic down for Lost World was really the best decision they could go with because if you at least have a good, sensible foundation that can be taken in a bunch of different directions, you'll be able to improve any future installments based on just that. That's how Mario did it, and it's something Jim even points out in the review you quoted - that they should keep going with this particular iteration of Sonic since it has the potential to grow into a bunch of different things rather than being level pack sequels.