Earlier quoted context omitted.
To buy Bitcoin, a miner somewhere has to produce a block with your transaction in it. If the value of Bitcoin falls that much, it will no longer be profitable to mine. With no miners, nobody could buy or sell anything.
The difficulty will be adjusted accordingly and automatically (it's part of the protocol). Your scenario can't happen.
Let's say the most recent difficulty adjustment has just passed, and the total network hash power suddenly drops by a factor of two. That means each block takes twice as long to find, so now you have to wait four weeks for it to correct itself. And during those four weeks, if the hash rate continues to drop, the adjustment gets further and further away.
Basically, if hash power drops by 5% per day for a sustained period, the network can't recover fast enough.