So going with the whole overtaking BTC idea, a masternode would cost $1,000,000, and the person would get paid out a 45% interest rate on block rewards. That doesn't sound very sustainable.
Hell even right now a masternode effectively costs more than $20,000.
Perhaps you should familiarise yourself with the concept of Scalability

Also the dash network can vote to reduce the number of dash required to collateralise a masternode at any time. The masternodes are critical to the network so if the number of masternodes were insufficient due to being cost prohibitive, the network would vote to reduce the number required to 500 or 200 or whatever were reasonable at the time. That's the beauty of the dash governance system.
The reason masternodes are collateralised is to protect the network from Sybil attacks. Also to ensure that masternode owners have a staked interest in voting on the best decisions for the good of the network.