Sonic isn't about speed for free. You don't get to run with nothing in the way for the entire level. Maybe you've heard too much about 3D sonic.
Sonic is about momentum and physics, any speed you can get is in sections where you're allowed to for fun or for speed runs with memorization, but generally it's about platforming with physics and momentum, not running the entire level.
A platformer where all you do do is run fast the whole time is like fast food. Maybe tasty but lacks hugely in substance. That's why OG Sonic games still put obstacles, because it'd suck otherwise. You're rewarded with speed for memory or taking the correct path, or using momentum in the right areas/times. It's not for free, because then it'd be cheap.







