Bliz,
I am not a Network Engineer or a Software Engineer. I believe your statement regarding solving the problem not being possible isn’t entirely true. It may not be financially viable but it is definitely possible.
Take all the server hardware you just shut down auto-migrating the dead realms. Go into each server and rename it. Something like this:
Benediction–>Benediction 1
Shutdown server 1–>Benediction 2
Shutdown server 2–>Benediction 3
Shutdown server 3–>Benediction 4
Take the code you already have “cross realm networking code”. Copy it and rename it “server pop problem fixer” Deploy that code so it does cross realm between Benediction 1, 2, 3, and 4. Update the authentication / character servers to search for the lowest ping/load server upon login (Benediction 1, 2, 3, or 4). Log that player into that server. Allow “cross realm networking code” I mean “Server pop problem fixer code” to connect players. Group/Raid invites prompts players to transfer between Benediction 1, 2, 3, or 4 so groups can be together. Yes, each server will need layers turned on still.
Do this for all the mega servers. Let players go where they want.