But what about when mining becomes unprofitable? If nobody were mining, then the only time anyone would contribute hashing power to bitcoin transactions would be when you are making a transaction. You'd probably just leave your bitcoin client running until the transaction is completed and then turn it off, because there's no need to keep it running when you're not making/waiting for transactions.
Does this mean the bitcoin market would crash if everyone just stopped mining?
Yes it is the same question and in a different thread, but if all of the miners stopped mining, the difficulty would drop making it less time till you get a block.
Then when you start mining, you would get the transaction fees and help get the difficulty to the speed that your computer goes. So you would get your transaction processed and you would pick up transaction fees along the way.
Now to step back into reality... Bitcoin has just grown too big to have 0 miners at any period of time from 2012 onward. It might eventually die or it might keep going, which if it dies in terms of mining, chances are, you wouldn't need to get any transactions through (unless you are selling to the bitcoin collecter, which then he would probably run a miner so bitcoin would not be dead).
If the hashrate suddenly drops, it will be slow transactions until the difficulty recalculates. Then it is back as fast as it was.
LiquidCoin mining isn't profitable but yet it still has some dedicated miners, so there wouldn't be much of a chance that bitcoin would just stop all transactions and 0 hash/s