They should fight at 155,
Oh wait, they can't. Usman is just too big.
So, this is your answer.
Also, Khabib doesn't need to ''solidify'', he is not the one who got choked by someone like Caceres and get finished by a fighter like that, or he didn't tap to a mediocre fighter such as Matt Serra's strikes.
Khabib is the only undefeated, P4P No.1 champion, who earned more money than anyone except for Conor. He is also the most dominant one with having most 10-8 rounds.
Usman's training partner Gaethje also said no one under 185 beats Khabib so, Khabib doesn't need to prove anything.