This is a key contributing factor in Dash's amazing stability.  A currency needs to be stable so that merchants will accept it without fear of it losing value, and Dash achieves this - quite remarkably - with the help of the masternodes.  Don't forget that these coins are not locked up because masternode owners can sell their Dash at any time, but what tends to happen is that those coins are just bought and put back into masternodes.  It's a brilliant system of organic equilibrium.

Also don't forget that just because Bitcoin has 16.5 million coins doesn't mean all those 16.5 million are on the market.  There are a lot of HODLers out there.  It's just that in Dash HODLers don't just sit on their paper wallets; they contribute vital services to the network, and are rewarded for doing so.