We would better not compare boxers and MMA fighters payment. Two sports are way to different. Get same audience to UFC, same sponsors, same history (UFC is to young, boxing championships appeared in ~1700, simply wait till MMA passes 300 years gap and see how much would fighters get). Boxers must be paid more, their sport is way more dangerous than MMA.
We don't know if Islam will do this, but in my opinion this is two different situation and Khabib is much better than both of this fighters, but we can only test this in virtual games now.
https://www.lowkickmma.com/islam-makhachev-mother-wants-him-retire/
Khabib follows theory, that a fighter can have a peak form only once. That was one of the reasons he retired from MMA. He felt that his form will start to go down so he decided to leave unbeaten. In the interviews he has mentioned that 32-33 is the peak age of a fighter. He has retired at 32, Islam is 31. I think he (or Abdulmanap) has inspired same theory to Makhachev. Khabib had 3 title defences, Islam only 1. I think we will see 2 more Islams fights, he will turn 32 and he will repeat Khabibs deed.