These dumpers are only going to stop dumping when they believe that it is NO longer in their interest to dump and that they can NO longer get any potential advantage from dumping...
This exactly is the problem, dumping earns a lot of people a lot of money. Start with 1000 BTC, dump them @ $480. When panic sell starts (and it always start) buy back at $465. That leaves you with 1032 BTC from starting 1000. Rinse & repeat, this time starting with 1032, transform them to 1065 in the next dump. Repeat again... Dumpers don't care if BTC will get to $500, or $1.000, or $10.000 by the end of the this year, or the next year, or 2016, when BTC gets there they will have a lot more BTC without investing fresh fiat. All that they have to do is not make the mistake when to start selling, but they already have a lot earned on the exchanges to cover for some errors in that regard. Easy BTC. It's hard to argue that they should just leave the BTC price to soar. They know it will eventually soar, but the reasoning is why not earn some fresh BTC from the weak hands along the way?