That is not correct since the price of the coin affects the return way more then the number of master nodes. Say twice as many people ran master node and the price goes up twice as much then you are still making the same. So by saying 'as the number of masternodes increase the return decreases' is not correct.

Say four times as many people started master nodes and the price went up 10 times. Well, now you are making more then before with more master nodes online.

Say I went and bought 1000 DASH and now the price goes down and now I"m not making as much as I was on that master node so the price has way more to do with return than the number of master nodes.