...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...
At the moment interest rate is ~9%, so I doubt it will be 45% then 
edit: "45% interest rate
on block rewards" - that's correct
Is there any reason why masternodes require a certain amount of coins up front to operate? Like a legitimate technical reason dictating requiring $20,000?
You mean 1000 dash, because not so long ago you could have it for $4-$7 each.
1000 it's a collateral, so won't be able to operate thousands of masternodes for free and be able sybil attack the network.
If the price rises to btc parity, then masternodes and miners could vote for a decrease the amount needed.
Doesn't Peercoin's minting set up for POS avoid these issues and manage a much more reasonable return rate?