The Spindash is an iconic and super useful move for Sonic. When they added a bunch of moves for Mario to help him navigate in 3D, no one said "oh they make the game easier so it was a mistake". If you don't have momentum at the bottom of a ramp and or a loop, you do a quick spindash to get through. It's also great to get a quick boost when the level starts. I'd argue that the games are infinitely more fun with it, and not having it in sonic 1 is the mistake, albeit the levels aren't as well suited for it.