I suppose the next expansion in 1.5 years is quite possible. They'd give us another 5 levels, probably re-work 60-70 like they did with 1-60 this time around and throw in some new areas. The team that did Cataclysm simply switch to the next expansion right from Cataclysm release. WoW is so polished and fine tuned now that it really isn't a major drama for them to come up with enough content to justify an expansion in 1.5 years. They're so good at this now and they know exactly what they're doing.
It does however suggest that WoW only has 2 more expansions left within it's natural life before Blizzard hope we all move onto the next big thing. So enjoy WoW while you can.