Is it possible for ~all the available Dash to be used to form Masternodes? ie. what is there to limit the number of Masternodes created, other than the MN payments being reduced to almost zero? And obviously the cost of acquiring all the available Dash.
It is technically possible but it will never happen. Many people are just traders and don't want to keep nodes. There are people who keep dash in their wallets and are in long term but still they don't have masternodes. Also when the price rises some liquidate their nodes to sell and buy back lower.
Buying masternodes would increase Dash price a lot, making continuous buying of Masternodes increasingly prohibitive. Payments won't really be reduced significantly because said buying will also increase Dash value. Even if Masternodes were to receive less Dash/payment, fiat value/payment would still be quite high.
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.