I mean come on… Google has servers capable of having a billion visitors a day. I understand there are vast differences… but the technology exists to have more than 15k people connected to a single server at a time.
The issue is the way WoW is built, its not built to just “add more hardware” due to the way the AH, chat, grouping, etc is built.
It would take a decade and millions upon millions of dollars to fix. Its not feasible.