You don't seem to understand business at all, allow me to enlighten you a little.
A business cares not one bit about their market share in terms of sales. They care about profit, profit, profit.
Sure, if they dropped the bottom out on the prices for the 290x et. al. they would get a surge in sales, but they would make less profit per sale, enough to destroy their margin. They have to get as much money as they can, while they can. Market share be damned.
That was kinda the operating strategy of Sun Microsystems for awhile there. Just before they woke up and realized there was a noose around their necks and the stool they were standing on was down to its last leg.
Market share is interesting because if a company has low market share because it sells its brand as some kind of exclusive, expensive, "i'm better than you" aura (BMW, Apple, etc.), then they make a killing on profit margins.
But that's not AMD. AMD is like the restaurant that has cheap food and everytime you go in there, you notice the place is empty. People don't like to go to empty restaurants. They think there's something wrong with the joint. They want to go where everyone else is going.