As much as people enjoy grousing about them, CIG [0] has made some really interesting progress in pushing the boundaries in player counts. This article discusses this in a very hypothetical, back-of-the-envelope sort of way, but from what I can tell, CIG are actually making it work today using dynamic server meshing, database-like servers for managing global game state, and intermediate state replication layers for passing players and player actions between servers seamlessly. It's still a work in progress, but it's showing promise. Expensive, and the sort of effort other studios don't seem interested in investing in, but seemingly doable with today's tech, and seemingly no need for a global multicast network as the article suggests.
[0] https://cloudimperiumgames.com