Anyways, there is some kind of floor to which they can fall, and which will catch them.
What kind of "floor" is there for crypto? By definition it is not tied to anything non digital? Right?
Not a finance expert, but I disagree with what you're saying:
There's a feedback loop between:
1. The price of BTC, which depends on people's willingness to mine it (without which, BTC doesn't have security).
2. The willingness to mine BTC, which depends on the price. Why would you mine BTC if you're not sure whether you'll be able to sell it for more than you mined it for?
A decline in 1 can trigger a decline in 2, which can trigger a decline in 1, which can trigger a decline in 2.
BTC can have an end. Oh, and BTC is not like BitTorrent.
The "feedback loop" you describe would reach equilibrium due to difficulty adjustment (blocks could be really slow for some time). Granted it would then become cheaper to 51% attack. Maybe one could consider Bitcoin "dead" at that point but it would still be working.