If I had to guess, compared to other primates it takes human young too long to be any good for anything. Permanent breasts would make cross-nursing more feasible in the event that a mother died or couldn't produce enough during scarcity.

No sources or background, just throwing darts at a plausible "the ones who didn't died" scenario.

> Permanent breasts would make cross-nursing more feasible in the event that a mother died or couldn't produce enough during scarcity.

I don't understand what you're trying to say here. Most mammals do fine with nipples and no breasts, whether they're nursing or not. And women lactate after giving birth; they can't do it on command.

On the other hand, it doesn’t seem to be “oh no nudity” if a woman wears a very skimpy bra that only covers their nipples and fully exposes the rest of their breasts.

Most people do consider that nudity. It's usually done to work around laws, not to "work around" public opinion.

Men have breasts. Also, my "theory" doesn't need to explain why women have breasts?