From what I see, Blizzard is slowly rolling out everything they need to eventually get rid of realms entirely as separate technical entities.
Cross-realms features are everywhere, the concept of PvP servers is no more, and if they manage to upgrade the auction house system to something more global, realms will be mere namespaces and players will be able to seamlessly play with all other players from their region.
Cross-realm guilds would be a big one. I was in a mythic progression raiding guild during Legion and recruiting was always a mess. I didn't play on one of the "top" servers for raiding and there just weren't enough quality players to go around. You'd have a handful of solid guilds at the top and everyone else would struggle to complete a tier before the next tier came out. And then in the middle of the expansion when player counts drop it got even more difficult to recruit. If you made the mistake of creating all your characters on a "dead" server you'd be looking at several hundred dollars at least to transfer them all. Cross-realm guilds are a long time coming.
Yes, I didn't mention it in my post because I think the auction house and the underlying economy issues are going to be the hardest part in making everything cross-realm, but cross-realms guilds are highly desirable. Your arguments are on point.
Cross-realms features are everywhere, the concept of PvP servers is no more, and if they manage to upgrade the auction house system to something more global, realms will be mere namespaces and players will be able to seamlessly play with all other players from their region.