The "arena-tournament" private server has about 5-6k players at peak hours. and they are only an instant 60 WOTLK pvp server.
I am not going to argue that there arent any decent populated private servers...because as you pointed out there obviously are...I also know this...however, as i said, I highly doubt a X1 blizzlike private VANILLA server, will see those numbers, any time soon...there just arent enough people who enjoy the bare minimum leveling journey that blizzard had offered at one point. Most of us are here to experience that journey again/for the first time, however we are a small minority compared to the grand scheme of WoW