The problem that GSP is going to face is a) age and b) wear & tear on the body...
Everyone slows down with age, that is an absolute... some slow more than others... and to be frank, certain styles are more inclined to degrade fast then others... GSP has always relied on his speed, and agility... and remarkable conditioning... Slow him down a notch or two, and it is going to have a big impact... the good news for Georges, is that his is so dedicated to his craft, and meticulous with regards to training, he is not going to age as fast as a fighter who lives a more leisure inclined lifestyle... but after the age of 30, performance typically declines... after 35, it declines rapidly...
The other factor is the amount of wear and tear that 27 fights, with 20+ of those fights against top 5 competition, have had on his body and mind... add to that 2 ACL surgeries... it is quite possible that GSP is simply not be able to get back to a physical standard we have seen previously... The guy has taken a lot of damage the past few fights... perhaps time will heal a lot of the wear and tear his body has endured... but that is still a very big IF...
The other question that has to be asked... and the reason so many fighters come back, is the lure of one big final paycheck... sure... We all know that GSP has done very well over the years... and he appears like the guy who has taken care of his finances... but if a ridiculous offer were to be extended... that may also be a factor in bringing Georges back for a final run.. It wouldn't be implausible for the UFC to extend such an offer under the right circumstances, or environment... at the end of the day, GSP is the PPV king, and his brand is still well in tact...