I think the idea itself is brilliant (hey, I LOVE self-regulating systems), but it's not a good idea for Darkcoin.
Right now, an institution (Government, some billionaire, some agency) can buy whatever they want number of Masternodes and there are still "free" Masternodes owned by you and me to secure the anonymity.
If driven by market-force, they could buy-out EVERYONE else and get 100% control.
Great point. Just to add some numbers to it...
As an example, assume you have a pool of 1000 masternodes.
DRK Method: Evil rich entity would have to buy 4000 nodes (nearly impossible) to control 80% of the network.
Spread Method: Evil rich entity would have to buy 800 nodes and maintain a larger balance than 800 current owners to kick them out. Rich entity would have no problem keeping a high balance compared to Joe Normal.
Spread method is great for pumping the price, but it is plain which of these methods is a better security model.