Mythic raiding and guilds are crossrealm, realms shouldnt be an issue nowadays.
As for adding 10m back Im not saying it could never happen but they wouldnt get rid of 20m. It would require more dev time as they would have to strip mechanics that dont work with less players, it would be harder to play what you want like in m+ as there are fewer players and loot would be even worse to get in 10m. Overall there’s not much incentives I feel to do so when mythic raiding is pretty niche.
I do think adding hard modes to some heroic bosses might help to sate heroic raiders hunger for more content but mythic is a bit more complicated.