And all of those guys outside of Khabib are arguably better strikers than wrestlers in MMA. The most important aspect of MMA is subjective, but in it's rawest form would most likely be considered BJJ. Today it could be considered boxing.
BJJ > BJJ & Wrestling > BJJ & Wrestling & Muay Thai > BJJ & Wrestling & Muay Thai & Boxing.
In today's game, especially if you're at a high level it's mostly expected that you have good BJJ, solid Muay Thai, and solid wrestling. What is starting to separate the elite from the others though is just how well they move with their feet and their hands. Either to outright win with them, or set up their grappling. Even Khabib used a lot of this although not necessarily executed well, he has the advantage of experience which continually improves fight over fight.
I wouldn't say any of the four big martial arts are necessarily more important than the other, although because of the desperate need for boxing it should be the focus for most fighters. They are all essential in certain situations though.