Prestigious journals, assuming reasonable editors, can spontaneously emerge and attract submissions. Authors publish in journals based upon some combination of 1) editorial quality, 2) whether or not everyone else in their field publishes in it, and 3) prestige.

2 and 3 are self-reinforcing effects -- why does Nature attract prestigious papers? Because it publishes prestigious papers. Why does Nature attract high-quality research? Because it is full of high-quality research.

All of this is to say that it is likely that, given a distribution of reasonably-edited arXiv-overlay journals, there's a pretty good chance that just a few of them in any given field (or at large) will attract most of the serious work