I haven't been following the other game. Are there 2D stages there too?
Yeah, it's essentially Generations with mostly original stages plus an avatar character.
Before Mania I think a lot of people would have wanted that. But now Mania is here, having Classic there feels redundant - especially since the physics/level design don't seem to have improved much.
I sympathise with Sonic Team to an extent. I imagine Classic Sonic's role in Forces was set in stone far before Mania came to fruition. Even so, that's not an excuse for seemingly not improving it at all.