1) Besides Sonic Colours, Sonic Lost World and Sonic Unleashed (it had some really great stages), Sonic Gens is the only really good Sonic game in years. SA2 is a perfect example of an extremely flawed game yet beloved by some fans.
2) I liked Sonic Unleashed, Sonic Colours, Sonic Gens, Sonic Lost World, Sonic 4, Sonic 06 and even a few aspects of SA1 & SA2 so you're being overly pessimistic.
3) Sonic Gens nailed the 3D gameplay for a Sonic game while SLW has more of a Mario Galaxy traditional platforming style, so I've got to disagree completely.
4) And what's the problem with that? It's like complaining that Kid Goku shouldn't be playable in Xenoverse.
5) The great thing about Sonic Generations is that its stages have tons of replayability and if played on PC then there's loads of mods to use (Sonic 06, Sonic Unleashed stages and whatnot).