There are a couple of things at work in Dash with the difference.  First, if the hash rate suddenly increases, DGW will quickly adjust, even if they suddenly leave, and the hash rate drops, it only takes a few hours to get through tops, not 2 weeks like Bitcoin.  Second, the hash rate determines the payout.  We've been above the hash rate that allows us to be paid a little more than minimum almost since the beginning.  Sometimes we went below it and got a better payout, but it hasn't done that in a long time.  So even so ASICs will raise the hash rate, it won't drop the payout below minimum, which we've been at.

Evan has seen all this coming and even so I don't know what incentives he's planning that will keep centralization from happening, I'll bet he did find a solution.