Yah, like InTheWoods said, it becomes too prohibitively expensive. Since coins are always being produced, we can gain MNs at a steady rate, but I think we're close to maximum right now. It would not be desirable for so many MNs either, as we have a liquidity problem already. People don't want to trade their Dash which makes it hard to buy Dash, and when we get to the point where people are actually using Dash, we gotta hope they can buy and sell their Dash quickly and conveniently.