If all BTC are mined, the "miner" still get the transaction costs. So this could still motivate them. But due to the high transaction costs, it could be that BTC will be a store of value like gold in real life. There are many options like NEM, Litecoin, Dash etc.
What I believe right now is, that the blockchain technology will still survive and not like the most of the ppl are telling right now: It's a bubble and nothing else. The cryptocurrencies are just "virtual numbers" nothhing else...