If I were Conor I wouldn't want to fight Khabib either.
For his part, Conor asked for the biggest challenge available - the champion in RDA who was beating everyone up. You can't run through Diaz, Benson, Pettis and Cerrone consecutively without being called the best. Conor booked a fight with the best.
Khabib, Ferguson and Alvarez are all more difficult fights than Diaz. All three are also on meaningful win streaks and more deserving.
This is a holding fight though. A short notice replacement. The plan wasn't 'who is the second biggest challenge' the plan was to book a money fight since the title bout fell apart.
Conor will eventually have to fight all of them if he sticks around, so its really a moot point.