The quality in those titles are already abysmally low, regardless of adding classic gameplay or not. This can be blamed on rushed deadlines. QC teams work very hard but developers don't often get time to address the bazillion issues they may or may not find.

I'd venture to say Sonic Team really needs to find better ways to transition their gameplay philosophies from the old 2D games to the 3D ones. These newer games generally leave Sonic having an identity crisis. There should be an emphasis on speed and platforming.

Even as early as Sonic 1 on the Genesis, Marble Garden should be a lesson in level design for the franchise. It's slow and tedious without any pathways that open from skillful platforming. Sonic Team learned from this by making the subsequent games have better level design to compliment the speed and flow of the gameplay.

Fast forward two decades, and games like Sonic Unleashed have slow boring brawler combat. It shows me that Sonic Team really doesn't know what direction to take the series in, and unfortunately that's just one example from the vast lineup of titles. Even games like SA2B are guilty of this. Did anyone really enjoy hunting for pieces of the Master Emerald in that game? Heck, did the cutscenes from Adventure 1 age well, if at all?

I know I'm kinda cherry picking here, but 3D Sonic games have been very consistently underwhelming, for a multitude of reasons. Of what's out there, I had fun playing Colors, SA2B, and Generations, but I'll never touch the other titles. The games just simply aren't that great.

And don't get me started on Sonic 4. Cybershell's Youtube video(s) explain how awful those games are far more eloquently than I ever could.