Interesting. I thought the WoW team would've had this nailed down, since they've been doing it for 14 years. I wonder what came up.
Seems to be a fundamental flaw with their engine or the way the server runs the game. We already know that the engine has limited the developers capabilities, and that's there far, far too many assets and far too much code already built into the engine to abandon it for a new one. Looks like it's going to be what we are stuck with until the game's end, unless Blizzard has decided to invest in technology that can transfer the exact layout of their world, all of the assets, all of the items and character files over to their new engine, which TBH sounds borderline impossible. Not to mention, they'd have to completely rebuild all of their code improvements too.