Even if prime Lorca is still about somewhere, that won't be the same character, and that's a shame.
I am annoyed they went the Mirror Lorca route, I was hoping Trek would give us a character who was flawed, ambiguous and a bit of an anti hero without needing a supernatural 'explanation' for it.
Other than that though, I thought this episode was great. Very well presented and gripping, with some great character bits with Stamets and Culber, and Yeoh is fantastic as the Emperor. An almost totally different characters but with enough the same to ground the connection between her and Burnham.
I liked the unflinching brutality of the MU this week; this wasn't coy, campy evil with gold sashes and ambiguous sexuality played for laughs. This was a universe of real horrors, which you could believe would generate amoral monsters.
So the spore drive doesn't exist in the MU, and it appears Mirror!Stamets messing about with the tech may have created a disease or similar which is wiping out the network - sowing the seeds of writing out the drive from the show, I assume. It raises the question of whether that was, speaking fatalistically, inevitable - if the network spans all parallel universes, in one there was bound to be a Stamets who made that mistake.
One question that remains though is that we seem to have parallel stories going on between Lorca and Stamets. Mirror!Lorca wanted to go back to the MU, Mirror!Stamets was trying to contact Stamets from the other side; which was responsible for Discovery ending up here? Both? Did Mirror!Lorca know of the technology on his side so sought it out on our side and dragged it out of a lab and into service?