A couple of commenters mentioned on the previous post that having paladins be Alliance-only would mean that the Horde would have fewer tanks.
Has having more tank classes led to more tanks, though?
Vanilla only had 2 tank classes: warriors and bears. Now we have five. Yet it seems like the proportion of tanks out there is still the same. It's still very hard to find a tank, just as hard as it was in Vanilla. I am suspicious of the idea that death knights, monks, and the emergence of pally tanks has lead to more tanks overall.
Of course, it's also possible that there are more tanks now, but that Dungeon Finder and LFR introduced a larger population of DPS into regular grouping, which has swamped the increase in tanks.
But I don't think that is true. I think the number of people willing to be tanks is just rather low, regardless of the different ways one can tank. Introducing more tank classes just increases the diversity of tanks, but not the overall total.