I agree there are too many biased GSP fans who think that at age 40 he's still as good as he was at 28. I have no idea why anyone would think that of any athlete in any contact sport, but there you go.
However, the knee injury seems to be a pretty good reason for him not to come back, or to come back worse than before. Not all knee injuries are the same, not all operations are successful (if you Google you'll get a number around 80% success rate for ACL surgeries, so you expect 20% failure rate, making it very possible he's one of the 20% -- he sure seemed slower and with poorer footwork in his post-ACL fights). Which is another reason he should stay retired. An old guy, with one fight in six years and a knee injury that wasn't completely successful should not be fighting. He should definitely not be gifted another unearned title shot at any weight division. If he's really still good then let him prove it by working his way back up the rankings.