Finally someone brings this argument up. This is the one semi-legit argument that separates GSP from other cases. For example, DC is going to retire next year, and could possibly vacate both 205 and HW belts without defending them until he loses. But I don't think DC will get hate, because it's acceptable to retire at 40. GSP on the other hand is viewed to have retired "in his prime", which is distinctly different.
But allow me to shatter this argument for you. Firstly, in combat sports it's not just absolute age that matters, but career age. DC didn't even START fighting in MMA until he was 30 years old. GSP by contrast has been fighting in MMA since he was 20!! That means GSP's MMA career was already 12 years long at 32. He was worn out at 32.
Now, your counter argument is obviously going to be that 12 years isn't a long time. But another factor is the training grind: You've got guys like "Gumby" (Jeremy Horn) with 100+ MMA fights over a 20 year period!! However, Jeremy Horn didn't really train all that hard. Most of his fights were basically training, fighting easier competition. GSP most likely put in far more training hours over his 12 years in MMA (20-32 years of age) than Jeremy horn did in his entire career.
GSP was fighting the best of the best for most of his run at 170. That level of training, facing elite competition every fight, wears on a fighter. And training for 5 round fights is far more challenging than training for 3 round fights. At the age of 32 GSP had 2 major ACL reconstruction surgeries, and god knows how many other smaller injuries nagging him. You literally cannot keep that championship pace up for 10+ years. Just look at Mighty Mouse -- he basically shrugged his shoulders and walked away after losing to Cejudo. He's tired and needs some time off after such a gruelling championship run.
Basically, you cannot fathom how much of a toll GSP's career would have taken on him by the age of 32. Most guys don't reach that level of exhaustion until their late 30's.