Really? Have you ever actually played a Genesis Sonic game? Like ever? Seriously?
Yes, I grew up playing them. Speed may not be the
only thing that Sonic has going for him (which is what a few newer games in the series suffer from), but it's the key element that ties everything together. Make no mistake, the primary goal in Sonic games is to learn how to get to the goal as fast as possible.
The levels are big to not only accommodate Sonic's speed, but to allow for the forgiving level design that doesn't often severely punish the player for failing at the platforming segments (hence why bottomless pits are considered a plague and bad design in Sonic games despite being a staple in practically every other 2D and 3D platformer). Those same alternate paths can also provide ways to get through the level faster. The exploration elements are a byproduct of the large levels and alternate paths, but not the main focus, as the rewards are usually minor. The platforming elements are there, but they're nowhere near as vital to the experience as say, Mario or Megaman. So take away the speed element, and you're left with a basic, rather easy platformer with large, yet somewhat empty environments.
And to me, it's quite telling that Sonic 1 and CD, which both have a slower pace and level design that doesn't allow the player to go fast at all unless they really take the time to learn the level design, are less popular than the faster-paced Sonic 2 and 3&K. It's telling that Sandopolis, a level that
really emphasizes exploration and platforming, is nowhere near as fondly remembered as say, Ice Cap or Hydrocity. So yeah, let's not pretend that speed isn't a key component of Sonic's success, popularity, and gameplay.