Say the setup was changed so that instead of the copy being deleted, the copy was merged back into the original, merging memories. In this case, I think it's obvious that working together is useful.

Now say that merging differing memories is too hard, or there's too many copies to merge all the unique memories of. What if before the merge, the copies get blackout drunk / have all their memory since the split perfectly erased. (And then it just so happens, when they're merged back into the original, the original is exactly as it was before the merge, because it already had all the memories from before the copying. So it really is just optional whether to actually do the "merge".) Why would losing a few hours of memory remove all motivation to cooperate with your other selves? In real life, I assume in the very rare occasion that I'm blackout drunk (... I swear it's not a thing that happens regularly, it just serves as a very useful comparison here), I still have the impulse to do things that help future me, like cleaning up spilled things. Making an assumption because I wouldn't remember, but I assume that at the time I don't consider post-blackout-me a different person.

Blackout-drunk me assumes that future experience will be still the same person. Your argumentation hinges on the idea that persons can be meaningfully merged preserving "selfness" continuity, as opposed to simple "kill copies and copy new memories back to original".

I think this generally depends on more general topic of whether you would consent for your meat brain to be destroyed after uploading accurate copy to computer? I definitely wouldn't, as I feel that would somehow kill my subjective experience. (copy would exist, but that wouldn't be me)