To double the nodes is impossible at this point (more coins than now exists) and don't you think something might happen to the price along the way? The rise in valuation would be insane and there's no liquidity in the markets anyway, both factors limiting what an attacker could buy.
And the benefits would be hard to see, beyond saying that it would be "broken" in that case, but no REAL damage to the network. Whether or not the previous probabilities are correct, the system has been designed to make sure that bad actors can't hurt Dash.
To add 3500 new nodes yes you're right there is not enough supply, as of right now. However some of that money could easily be used to buy off current MN owners instead of creating new ones, or the money could be used to bring down some of the ones already operating. And they would not all be added at once anyway. As I mentioned earlier this would be done over time, possibly years to create the illusion of a healthy network. Yes the price would go up but by how much? Even at $50 per Dash the costs are not prohibitive. And when the price goes up, the amount they receive in return from dumping their generated Dash would go up as well. Not to mention this creates sell pressure helping to keep the price down.
I would also add that the low supply of Dash places a fundamental limit on the number of masternodes. Which means an attacker needs to acquire fewer MNs. I think this is something nobody has really considered. Earlier somebody made a comment about the Dash MN network being more decentralized over time. I think its going to be the complete opposite. It will be harder and harder for people to run their own MN over time simply because of scarcity, thus consolidating the network into a handful of people/entities. Its the centralized pool problem all over again.