Actually, I'm 100% positive that MoP increased the number of tanks - you only need to put yourself in a queue to prove that. Healers, I'd estimate remained about the same or increased slightly as well. Both can be attributed to the ease of gearing up, more focus on their DPS contribution, and the proliferation of the LFR and random-queue system, which makes people want to take the option that gives them the shortest queue time more often.
WoD will probably increase each further, because getting gear for the role is significantly easier without having to have specialized gear, and by making gear change primary stats such that those with DPS specs with different ones (Enhance, Ret, etc) will be able to gear up as easily as same-primary offspecs (Elemental, Balance, etc).
Maybe we have seen, and will continue to see fewer tanks and healers out in the world - because the same works the opposite way too (why quest as a Holy Paladin when you can just grab a 2h, keep your normal gear on, and the majority of your gearing for switching to Ret is done?) - but that doesn't mean there will be fewer tanks or healers for situations where tanks and healers are good to have.