Nobody can beat him unless maybe some of Khabib students or cousins, I think he would have a lot of problems with their style of fighting.
I think I saw Khamzat Chimaev saying he can beat Adesanya, but first he needs to get up one division from welterweight.
I don't think betting on martial arts is popular either. There is more money in betting on the more famous sports such as Football, Hockey and tennis. I've never researched it though. In martial arts it is difficult to determine who will win because you depend on 1 player and the form of the day. And if a fighter has slept badly or is not feeling well, it has a huge influence on the offered odds.