It does seem to be quite an age difference. GSP himself mentioned that his age (39 years old) might compromise his cardio and style against Khabib (32 years old). Given that Khabib managed to tire the hell out of one of the best 155'ers in their prime years in the first 1-3 rounds (Gaethje was depleted as hell just in the 1st round), i'd reckon the match might devaforize GSP by quite a margin.
I feel like a complete Lightweight legacy would've been fulfilled if Ferguson was the 30-0 finale of his career and the perfect end to it. The matchup that got cancelled 5 times throughout the years and when they finally managed to make it, a pandemic occured and got cancelled again. Even then i wouldn't give too many chances to Ferguson, but it would still be a great stylistic matchup.
Either way i don't see Khabib returning, unlike most fighers in the UFC, he doesn't seem the type to "retire" then to come back again. He promised his mom that this is the last fight. One can hope but slim chances for that.