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.
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.
Say Bitcoin crashes down to $1 tomorrow, such that it's no longer profitable to mine. A bunch of miners go dark. What happens to the block time?
Now remember that difficulty readjusts every 2016 blocks. If there are now 1% as many miners, that means a 1000-minute blocktime and potentially up to 1400 days per adjustment period (average of 700).
In this situation, the only option would be a hardfork to bootstrap the network back to a live state, either by directly adjusting the difficulty or by readjusting more frequently and more aggressively.
And I feel like people for example aren't talking about the fact that technology doesn't really go away. Like any technology we've ever invented is still used somewhere. This will very much be like bitcoin.