Everyone's talking about this fight as the biggest fight of all time, and I get it. You have the returning "champ champ" after a long layoff, fighting the undefeated Russian monster and current champ. It's certainly the fight to make and one everyone's been pining for.
Nevertheless, I don't think this ones going to break any records. The momentum that Conor built leading up to, and even after, the Mayweather fight, has slowed. McGregor is still tremendously popular, but it remains to be seen whether all of those casuals still have the interest. Remember, it's the CASUALS, that break PPV cards into the stratosphere.
Additionally, the Conor-Diaz 2 fight had a built in storyline. Diaz is a comic book character, and was the first fighter in the UFC to beat Conor. Everyone (EVERYONE) wondered what would happen in that second fight.
Khabib, while super popular among hardcore fans and his Russian base, isn't as well known as Diaz was after that first fight, and certainly doesn't have the attitude or bravado of Diaz (although his unique trash talking game is still pretty dope)
Look no further than Conor-Alvarez. That didn't come close to touching Conor-Diaz's numbers and that was historic. First fight for 2 simultaneous titles in the UFC AND first Madison Square Garden card, HEAVILY promoted.