First of all, let me make a small clarification.
They didn't NERF your GEAR, they nerfed a specc.
There is NO Specific gear for WW Spec, you can use the gear for WW the same way for EVERY OTHER SPEC (Assuming people want Defensive or Offensive builds, but that's a whole new story)
My point is; what does the WW spec need? Crit? Crit DMG? Str? LOH? It's usable for each and every other offensive build out there. So people who bought gear with Real Money and insane amounts of Gold will NOT be burned, the gear will still serve them a great purpose. Other thing is that this nerf will affect only bad players or players who did not invest much time in the game. People with good gear will hardly feel the difference.
So to your original question of "How many will quit" I'd say 99% of the scrubs who roll on the "What's imba now" train will quit or roll another char and 99% of the dedicated and decent Barbarian players will remain.
Blizzard made a great move with this patch, the majority of people who rolled a WW Barbarian are bad players in general, they rolled it for the sake of the spec itself and not the Barbarian as a whole, aiming for a cheap and fast way to profit with very low expenses to cover on gear. With a mere 10 million gold you could do an amazing job at farming Act III, even if you're very low skilled and downright bad. - My point is that this nerf will only hit players who invested little to none in their gear, which basically means that Blizzard are hitting people who are pretty much worthless to them. They do not buy from the RMAH, thus they do not contribute to the company. They still farm slow and ineffectively, which on its behalf spells that they have low sales on the RMAH, if any.
And to the people saying it's "Just a game", "It's single player, how does that affect you?", "What do you care?" - As long as there are real money involved in it as well as a well thought and integrated economy, then yes, it does affect me and every other player in this game by a great deal.
To the people who say "We won't be able to clear Act 3 as fast now" - Well, you'll just get on the level of all of the other classes now.
So, in light of this upcoming nerf, I'd say that Blizzard made a damn brilliant move, they will not lose any of the players who actually care and contribute to the game. This, along with the other very fun changes in 1.05 will make this patch a huge success, just as its previous one, the 1.04 one was.
Good job at improving the game, Blizzard, keep it going! You have a great base to build on, This will be the game of 2013.
Edited by Vengoto#2574 on 22/09/2012 16:06 BST