Realistically? I think the mirror universe would be unsustainable.
Exactly. It certainly wouldn't be sustainable according to the principles that govern the Prime Universe.
Looking at it from another point of view, one of the problems that none of the MU sequels ever grasped so to speak was that the MU made the most sense only in the context of the events of Mirror, Mirror
involving the Halkans. The MU was a reflection of the Prime Universe, centered at those events, with the Halkans themselves as a fixed point, essentially identical in both universes.
That inversion makes sense from a literary perspective, to provide a pulpy "what if" scenario: What if the Federation took dilithium crystals by force?
Until the MU sequels came upon us, I always interpreted Scotty's technobabble about how the local field density between the universes was increasing to mean that the two universes were diverging, and once they had separated too far, they would never be similar enough to transport between again. The flip side of that was that the two universes had never been similar enough to transport between before, either, until they coincided closely enough in their respective Halkan incidents.
This whole business with the two universes having similar characters in various centuries, oh that just happened to be our main characters, never minding that most of the other people around them must have been dying off out of synchronization, if not completely different, threw any last vestige of sensibility out the window, as far as I'm concerned. The DS9 MU episodes are just gratuitous garbage.
ENT's IAMD was at least fun by comparison, warranting multiple fangirly squees. And its opening theme rocked.
To reconcile all that, we're back to our currents in time idea from The City on the Edge of Forever
and Star Trek
(2009). Somehow, both sweeping historical trends and important individuals in history occur simultaneously in both universes.
And this is the problem with revisiting the MU: day to day life in both
universes is irrelevant, in the sense that individual choices don't alter the flow of history; the two universes are fated to stay synchronized as Bizarro-like versions of each other. Your day to day life is just whatever realizes that fate.
And this implies that human nature is fundamentally different in the MU. The two universes just couldn't stay synchronized otherwise. Additionally, assuming that the Prime Universe is logical, there can't ultimately be what we consider logic in the MU; in the MU there can be only a Bizarro kind of logic.