I started playing in august.My first char was rogue, got him to lvl 70 in TBC, couldn't get a decent guild because all decent guilds were looking for paladins/warlocks/shadowpriests, so i abandoned him.
I has a warlock sitting around his 30's, so i leveled him to his current level (61).I changed playstyles constantly, almost every 10 levels, because of new talents and I was leveling with affliction only.

30-40 Voidwalker, simply DoT, send in the blueberry, DoT next, draintank and loot.

40-50 Succy, DoT mob, DoT next one, drain first, drain second, put some drain soul to recover mana inbetween, loot.

50-60 Imp, UA needs a lot more mana, but succy just couldn't generate it fast enough, so i switched to imp and even then, every missed drain soul was noticeable.But i was grinding 2-3 mobs at once and they were dying in 15-18 seconds every time.

I respecced to demonology just to try it briefly in outland and promptly respecced back, because i was dying pretty often, same think was happening to me every time i was switching pets. And i can only guess that desto levels differently too.
Soo, that's about 4 different styles i tried just by leveling my character ? I doubt someone who doesn't read every possible bit of advice like me would want to go through that, and i didn't even touch pvp...

The phenomenon of class distribution not following the numbers according to the flavor of the month is not suprising to me. I think it is the natural response to blizzard having such a high barrier to entry in the form of leveling, repgrinds, keying, and gearing.

If you look at games with less of a barrier of entry than WoW, you find that your expectations match much more closely. In Guildwars, it takes at most 24 hours and 300g (rough WoW money farming time equivelent) to get a capped level character that is just as good as any other person's main in one specific build (although it is severely lacking in the versatility someone's main would have). What you find as a result of this is that the flavor of the month is routinely flooded with new players rolling up and trying out the class. Since it is a PvP based game with decently hard counters, you then find that people start to then figure out ways to defeat this "overpowered" class and their counter becomes the new flavor of the month. (Note: most of this doesn't actually go on in the individual character level, but the arena team level, where entire arena teams will change thier builds with new characters being rolled up to suit whatever build the team has decided on). I see no reason why WoW wouldn't have a similar result if it didn't take so long to level/gear/faction an alt.

The reason why 'the market' of players as a whole isn't changing to take advantage of the overpoweredness is the same reason we all don't reroll when we see something overpowered come up: it takes too long, and by the time you get as strong as you were on your old character, it'll probably be nerfed anyways.