With less server population on WoW means less WoW dungeons & instances and slower BGs, lets face it, it's hard enough to get a good Naxx25 group/guild! Even WoWs game director, Tom Chilton
said
“I definitely do think there will be some MMO at some point that beats WoW, that's bigger than WoW,” he said. “Hopefully we're the ones that make it, but whether that happens, I don't know.” In February 2009 WoW game director Jeffery Kaplan announced he shifted positions from WoW to the unannounced MMO.
http://www.videogamer.com/news/blizzard_s_unannounced_mmo_in_development_for_years.html
Seriously if Blizzard were to do the likes of shut down WoWs servers or even completely nerf the population, can you imagine the suicides :O!