Agree with you that it could be fixed but it won't. The only reason I am seeing increased revenue is mostly from video. Otherwise I would be in the downward slide. Agencies, especially public ally traded agencies, can't give raises without justifying those raises to their stockholders on how that provides margin leverage or revenue lift. Istock is you know will never give raises and neither will Fotolia. These agencies are good at creatively cutting our royalties. I am not telling you anything you don't already know.
Actually, iStock went back to 20% for illustrators, so there is some hope. For the most part though, I agree. They aren't budging and don't have much pressure on them. I got lucky and have moved a lot of my business to smaller illustration sites. Still, I find it hard to completely eliminate the big players (even if they account for a smaller percentage of my earnings).