This idea has been suggested. There are several pros and cons. Because miners are ultimately making a bet on Dash, they have significant interest in seeing Dash succeed, not just masternode operators. The approach we've suggested in which the cost of proposals reduces both masternode and miner rewards proportionately helps ensure that beneficial proposals are more likely to receive approval. If you required the full cost of the proposal to come exclusively from the masternode portion of the reward, yet MNOs only receive a portion of the expected benefit (which accrues to miners, users, and masternodes), that creates misaligned incentives. It does introduce an additional source of variation for miners, but this is minor compared to the price uncertainty over the life of the miner's equipment. At the same time, it reduces the variation for the masternodes and protects against the risk of passive MNO investors selling in large numbers due to a sustained period of heavier proposal spending. Given that 60% of the proposal cost would come from masternodes under the proposed plan, that should be plenty of incentive to scrutinize proposals. As designed, MNOs are expected to provide 60% of the cost, yet receive less than 50% of any benefit (since less than 50% of coins are MN collateral)... that should align their interests with the networks better than requiring them to fund 100% of the cost.
