Why? Because MoP is going to come out and there are going to be a ton of Monks (the new class of the expansion). All of them are going to be rolling for leather and a Druid is going to be caught in the pack, fighting for the same gear. Until they get mail, Shaman and Hunters will also be using leather, so those first 40 levels could be ugly.
Except you're forgetting Warriors already have high competition with Paladins (the most played class) and Death Knights with their drops. Strength plate always seems to be fought over.
I also don't believe Monks are going to be overplayed like DKs were in WotLK. They're overplayed in Beta, but that's because many are trying them out, but I don't see anyone but the people that really want to play a monk leveling them from 1-90 on launch especially if they aren't overpowered which so far only the Mistweaver sounds anything remotely OP). Not that it can't change, but I feel a lot of people will get bored with them as soon as they hit Outlands.
That said, Warriors look like they are going to get a lot of cool stuff come MoP that seems really fun to try out. But, only a druid can fulfill all four roles in the game so you can't pick a better class for that. The problem is their PvP viability is all over. Feral is good for arena, but only Balance seems to be needed for RBGs. If you want to just play a druid in all variations of the game, you'd have to learn to play more than one specialization.