In traditional Star Trek fashion, Kirk makes an impassioned speech to Mirror Spock (the one with the iconic goatee) about the need to reform such an empire, if only because all this fascism was doomed to collapse under its weight eventually. Surprisingly, Deep Space Nine followed up on the Mirror Universe, and while we still never got a look at this dimension’s Earth, we did discover that the Empire had collapsed.
In a dark twist, it collapsed specifically because Mirror Spock ended up successfully reforming the Terran Empire, leaving it weak and vulnerable to an unholy Klingon/Cardassian alliance.