You see, in the same vein that you believe past transgressions means they shouldn't bother, I believe their past successes mean that there's still much potential to be unlocked. The amazing game that Sonic Adventure can be improved upon, and turned in to something seriously epic, but it won't happen if they keep churning out safe-zone 2D games and racetrack-boost 3D Sonics. They need to take a chance, they need to be allowed to take one.
Maybe that is what this anniversary game in the works really is, a return to the Speed-gameplay of Adventure but taking notes from the original mainly (Sonic, Tails, Knuckles and Amy being playable and only having very slight differences control/gameplay wise).
I would be hopeful, but a game like 06 shows that if you try doing everything at once, everything suffers. If they just focused ONLY on Sonic, Silver and Shadow and nothing else (no hub missions, amego characters, story), we could of gotten a decent game.
I don't want a SA3 now....but I would be open to one if they handled it like Unleashed. I was one of the crazy people that enjoyed the Werehog portions of the game and it was what I expected from Sonic Team; focus on two style instead of six or three leading both being quite polished and enjoyable. Same thing could be applied to Sonic Generations with Modern/Classic Sonic too.
I will give them a chance if the box says 'Sonic Adventure 3', I won't be excited, I will be worried until proven wrong.