Masternode owners will only vote for what's good for Dash because they're long term holders and want their investment to increase. How can they do that? By growing Dash.
Example, Greedy MN owners want to take more of the block reward for themselves so they vote to increase their share to 80/10/10 allowing all the miners to leave. Now the security of the hash has been compromised, compromising the system, users leave in droves and the whole network collapses.
Yes, that scenario would be a foolish move on the MN owner's part. Right now there are several proposals out there which will do just as you say, to grow Dash. It's a very fun thing to be a part of. Dash Nation is growing, and we will succeed.