I just saw the Weasle make a similar point. If Jose Aldo retired after his 13th UFC/WEC fight, he'd be retiring after smoking Korean Zombie (and that'd be his 7th title defense between WEC/UFC, fifth in the UFC).
The all time list is for sustained dominance and I mean dominating at the top. Guys like GSP, Silva, Jones, and Mighty Mouse cleared out their divisions and kept dominating as new generations entered that trained their pro careers with a bullseye on those champs. Those champs had ridiculous amounts of footage and styles of opponents to scout and prepare gameplans off. Khabib got out before that kind of adversity and it means his record has no blemishes, but it also makes his dominance less impressive because I've seen that loop with a bunch of guys, except those other guys kept going until weaknesses were found and exploited.
Also...I value Strength of Schedule over everything in the discussion. How you win being next. Khabib is closer to the way I value Cain Velasquez...Cain at peak was probably the best HW I've ever seen, but injuries took him out before he could establish it. Khabib is the best LW to me, but I drop him down the P4P list because he took himself out of the game before he could establish more.