assuming P != NP

Very weird to condition this market on that. OK, I see what you mean. A proof of "P!=NP => factorization is not NP-complete" suffices, right?

Related market I made: https://manifold.markets/FedorBeets/will-someone-find-an-efficient-algo

I don't get how this market could have a higher market price than this one: https://manifold.markets/IsaacKing/can-npcomplete-problems-be-solved-i. Given that integer factorization is in BQP. Giving a reduction of it to an np-complete problem would be equivalent to NP ⊆ BQP if I am not missing something.

@TassiloNeubauer In principle, even if BQP \supseteq NP, that other market could still resolve to NO if it were proven that our universe does not support the construction of a scalable quantum computer.

But no, you're not missing anything, this market was grossly overpriced.