If mining stopped completely there would be no bitcoin. Bitcoin is the mining. But even one person on a cpu could continue to mine. And my guess is that even if bitcoin fails completely, there will be a few people that keep mining for the next few decades.
Right, only one miner is actually needed. With less people mining, the difficulty drops accordingly. The only way for bitcoin to die with >=1 miners still active would be if every single other person mining stopped at once, and that remaining person couldn't ever finish enough blocks to trigger the difficulty to drop. The idea of which is of course rather absurd.