As Spinks rightly points out, players will be using the Cataclysm previews and beta to select which class to "main" up to 85. I know I won't be switching from my DK because I couldn't give up the Rusted Proto, but such previews have influenced my choices in the past and clearly have a noticeable effect on the WoW population. Honestly, I'd chalk up the lack of Rogues throughout this entire expansion as about 80% due to how unexciting their new abilities and talents seemed way back in Wrath beta. The other classes were just too shiny. The other 20% is that Paladins, Warriors, and Death Knights were equally strong melee classes that could also tank/heal (and many Rogue players switched to DKs because they were the cool new shiny), drawing Rogue players (including me) away.
Now the low numbers of Warlocks? 90% due to their outlook before Wrath hit, easily.
What each class gets going into the expansion defines what class distributions will be like for at least the following two years.
Once all the previews are here, I'll whip out my infamous Crystal Ball of Totally Dependable Except When I Guess Wrong Predictions and tell you what I expect the class distribution to look like a month after Cata is released. In the meantime, I have to comment directly on the Warrior preview that was released this morning:
How utterly disappointing. Yes, the rage change is perfectly executed, but it's a mechanics change. It's got no "cool" factor. The new abilities aren't exciting. The talent tree changes aren't exciting. Heroic Leap is kind of cool, but it's not enough to carry the entire class.
Based on this preview, I will be shocked if there is not a mass exodus from the warrior class come Cataclysm. There will still be many who love the flavor of the class and enjoy their warrior, of course.
And I'll likely still go forward with my Worgen Warrior plans - Titan's Grip Werewolf, HOOOOOOO!
I don't expect Warriors to die out at all. But I expect the population to cut in half, going from one of the most populous classes to one of the least.