This is how I see things right now and how I think (read fear) things will be in WoD.
The gameplay of Mists of Pandaria had some influence on the decreasing number of Tanks and healers because:
- Daily questing is most efficient as dps
- Scenario's favour (mostly) 3x dps
- There were very few 5 man dungeons between 86-90
- Hardly any reason to run heroic 5 man dungeons at 90
- Playing a Tank in LFR puts a target on you for frustrated people.
Now in WoD, I expect less reason to play as a tank or healer because:
- All of the things mentioned for MoP
- No flying, so world pvp will ensue and you will quest as dps.
- EVEN FEWER dungeons! (yet 5 more levels to bridge).
- You will have to fight for nodes/herbs more often because you can't fly (or fly away).
- Giant world pvp zone will not favour Tanking in general.
I know that we have double specs and that gear will be more usuable outside of main specs, but I still don't think that people will tank or heal because it's outside of their "comfort zone", and not unimportant either they might not be good at it because they don't regurlary play it because of the reasons above.
I will still play my Tank simply because I love tanking. But I gotta say, my current experiences and the foresight in WoD worry me somewhat.