Crackdude, you of all people know my extreme praise of Sonic Adventure. Of course I know what it achieved, but it was still not on the level of what the original trilogy did.
On graphics, Sonic Adventure 1 and 2 were incredibly ahead of their time, but with all things like that, they aged. However, it still took like four years with the release of Jak and Daxter to have a game that was above the level of Sonic Adventure in pretty much all regards, graphically at least. But for comparison, Sonic 1 still to this day remains one of the best looking 16 bit games. There were 2D games coming out TEN YEARS after Sonic 1 that did not look as good as it, and it can be argued that no Game Boy Advance game looked as good as this did. I would say that is Shenmue-level impressive.
When talking on engines, Sonic Adventure 1 and 2 used two different engines, but Sonic 1, 2 and 3 were all based on one another with minor tweaks. Elements of it were still used all the way up until NiGHTS and Burning Rangers, and when control is concerned, Sonic 1, 2 and 3 might be the most "Solid controlling" 2D games in existence, when we all know about people complaining about falling through the levels and such in the early 3D games.
I could literally go on for hours, but I am honestly not sure why any of it matters, I was just pointing out what the series has achieved, not how good each release is. I actually prefer Sonic Adventure to Sonic the Hedgehog 2, but I do not think it is a better product overall in any way.