You have to have some terminal values for it to make any sense, but that doesn't imply virtue ethics, which is a mistake you seem to be making in the OP.

I'm not even trying to imply anything about virtue ethics in the OP :-/.

What would prevent it from collapsing in a multiple-agent universe?

I think that our concept of morality as distinct from aesthetics seems to be primarily a social thing. Morality is about how we handle other people, or at least some abstracted sense of an Other with real agency. People, certain animals, Nature, and God are thus all considered valid subjects for human morality to deal with, but we usually have no moral intuitions or even deductions about, say, paper-clips or boot laces as such.

A religious person might care that God legislates a proper order for tying your boot laces (it's left shoe followed by right shoe, halahically speaking ;-)), but even they don't normally have a preexisting, terminal moral value over the bootlaces themselves.

So, to sum up the unpacking, I think that on a psychological level, morality is fundamentally concerned with other people/agents and their treatment, it's a social function.