I think that's the trick though. You can complete most of the classic Sonic games fairly quickly really, but people still go back and replay them to find new shortcuts, items etc and have fun. It's the same with Colours and Generations when you look at them.
I think SA1/2 in a way has fallen victim to the changes in the industry. In the 90s, all types of platformers were flying around and getting success commercially and from the media. Despite the criticism they receive now, both SA1 and 2 got good reviewers on their initial release, but during the early 00s, the industry seemed to be changing quite rapidly as the technology got better and people seemed to want games that really show it off like FF10 and MGS2 - SEGA'S equivalent in this was probably Shenmue.
As this was going on, platformers seem to start drying up and by the 360/PS3 gen, we had Mario and Sonic with it's old fanbase and the odd title here and there. Heck, even Rare couldn't get platformers to sell during this time. The rise of the indie scene where smaller projects can thrive seems to have changed that though.
Time hasn't been to kind to SA 1/2, but back on release, the industry was a different one to what it is now. It's worth remembering that.