Nay. Sonic's appeal is not only from speed. Sonic is about a lot of things. The Night stages added a lot to the game and had an enormous amount of moves to use. The combat was fun and there were nice hidden objects around the levels. There was a lot of replay value with upgrades and the whole idea added a new dimension to Sonic gameplay. The Day stages alone make for an okay, but simplistic game. It needs more than just the Day gameplay. Sonic deserves better than that as a franchise.
Sonic's appeal IS his speedy nature. SEGA tried to vary the formula drastically ever since Sonic entered the 3D world, and look at all of the terrible games that were developed. The upcoming Sonic Generations focuses entirely on speed, and that is why it's going to be his greatest 3D game, if not one of the greatest Sonic the Hedgehog games ever.
"Look at all the terrible games that were developed" in your view. The reviews from each individual game on this site shows multiple people enjoyed each new Sonic game with an exception with the 2006 title. Sonic's appeal for many fans including myself is beyond speed and SEGA itself wanted more for Sonic than just speed. How do I know this? Just look at how long they persisted before giving up. Generations has speed right, but unlike previous Sonic games, won't have the trademark variety which was a big part of the quality of Sonic games. If the previous and 'terrible' Sonic games weren't a part of the Sonic franchise, they would garner very little hatred at all and some would even be considered classics. But since Sonic is in the title, people expect it to be a certain way and any deviation from that certain path is bad in quality. Even the Genesis games had platforming as well as speed, and speed was added as a gimmick to make it different from Mario who was their main competitor. Sonic is and was always meant to be, by the developers' intentions, more than just speed. Speed is a big part of the core, but was never the sole aspect.