The people being specially happy about Mania were not exactly waiting for this game with open arms.
People were up for it considering a lot seemed to actually like Generations a lot, they didn't sour on it till more was revealed and it looked like trash. The game is out, it's trash. Mania being as great as it was certainly didn't help it.
But that is the thing, I doubt Sonic Team was expecting any goodwill to say "let's realese it this year to get more sales". This is just the quality that Sonic Team and Sega are willing to accept outside of stuff like Mania.