I've joked about annihilating Mandalore in the past
As a Mandalorian writer, and NEO staff, I am
begging that someone takes a crack at it, I promise you people would not care nearly as much as you think they would nor would the fallout impact the way people write a Mandalorian. The Enclave did perfectly fine on its own despite not being in any of the Mandalorian core worlds, because it never mattered where Mandalorians are, they're everywhere. Annihilations are great for creating benchmarks and canon events to base a character's history/roleplay around.
We don't see a Chiss Ascendancy faction just because not that many people write a Chiss to begin with, not because Csilla is now a mote of dust in space. And assuming we did, the destruction of Csilla is already a premium motivator for anyone willing to put forth the effort and getting the writers needed to reestablish the Ascendancy. Dathomiri witches if Dathomir got blown up, can still very much write their voodoo wherever they want. It adds depth, and while I do sympathize with the loss of losing big name worlds like Coruscant, Mandalore, etc., the Annihilations are there to incentivize being a part of a tangible piece of Chaos' unique timeline and universe, like showing up at the Battle of Coruscant in Ep3 or something like that.
But going back to the original point as Val stated it would only be a matter of who 'owns' the planet or in this case wouldn't own, making Annihilated hexes impassible does literally nothing to lock people out of roleplay, this would only affect the nature of the map game on the scope of how people would have to play around it from a mechanics point of view which is a problem for Major factions and their staff to figure out. If I understand Val's suggestion correctly, this has literally zero impact on the individual or even minor faction level. You want to write a thread visiting the ashes of your now gone homeworld, nothing would be stopping you from putting a thread up and doing just that.