@Panda:
Even if you accept your latter statement as given fact, what does that have to do with Law betraying the Straw Hats, at all? They are currently in an alliance for the explicit purpose of defeating Kaido. Once that has been achieved, the alliance can end, they can go back to being enemies of each other, and nobody will have betrayed anybody. Saying Law will betray them means that he will take action against them while they are still allies. While that's obviously a possibility, I honestly don't see it as very likely, and would really need to ask you what sort of justification you'd have for thinking that it is something that will 'most definitely' happen.
I can't imagine a simple declaration of their pirate alliance ending happening for multiple reasons. First off ending the alliance that way with a line such as "The next time we meet we're enemies" would neuter any confrontations the SH and Heart pirates have in the future by rendering them devoid of emotional motivation, a fight between friendly rivals vs. actual enemies.It would make any fight between Luffy & Law in which they're both going all out much less believable and less satisfying, how could Luffy go all out against someone who'd basically be a friend at this point?
Equally important is the fact that what you and others are suggesting contradicts part of Law's characterisation. Law is a smart, calculating individual whose every move from becoming a Warlord to allying with Luffy were designed to advance his own goals, hell he probably only saved Luffy at the war so that he'd have him as a willing ally for the future. A smart, calculating individual wouldn't simply declare the alliance at an end, instead they would accomplish the goal of the alliance then strike their partner when they're vulnerable.
I think Oda is going to use the alliance arcs to build up Luffy and the SH as a serious force to be reckoned with in the NW. Right now they're unknowingly building their fleet and I feel like Law is going to be defeated by Dofla and Luffy will ultimately take him down which will be a major showing and make Law see and respect Luffy as a serious threat for PK. And smart, calculating characters deal with threats intelligently.
My only seed of doubt was Law seemingly honorably saying he was allied with the SH to Fuji but that couldve been done to endear us to Law and make any betrayal sting more.