It's not that clear-cut. By Star Trek: Coda rules, the First Splinter's own mirror universe was tethered to the First Splinter despite nominally being spawned from a different sector of the multiverse entirely. Even the Captain Riker universe from Headlong Flight was a splinter attacked by the Devidians despite diverging in history before 2373.Well the Myriad Universe stories were alternate universes, so why would they cease?