I don't think people are realizing the true ramifications of this episode. I believe people may be thinking way too small. Forget just visiting the mirror universe, Discovery can travel to pretty much ANY timeline or any parallel universe now. The Kelvin-verse, the mirrorverse, the past, the future.
It's possible this was never the Prime Universe as we knew it to be, but one version of it.
It's possible this was never the Prime Universe as we knew it to be, but one version of it.