I think Cain could definitely make LHW but even if he did Jones would make him look terrible. Jones is just a monster.
No need to worry though Cain because even if you lose Overeem will be smashing JDS and then Cain will still need to fight Overeem for another shot at the belt....but then Overeem will beat Cain too....then Cain will need to drop down or be content at being the #3 HW in the division...that is until Jones steps up and Cormier comes over.
JDS had a great UFC run but right now he has exactly one title defence against and his most difficult fights agead of him. He is not some kind of unbeatable champ like GSP or Spider yet.
Even if Cain gets knocked out in the first again, by the time he builds up a streak to be back for contention title may very vell swap hands again. Cain looks good at HW. He shouldn't try to fix what isn't broken.
Cain carries at least 10-15 pounds of unnecessary fat and he fights around 240. So for sure he could diet down a bit and cut water to get to 205. But does he need to? I don't think so, he proved against Lensar that size wasn't a big issue for him.