How low would Bitcoin have to fall before it was no longer recoverable?
Why do you expect this?
As long as there are many people who hold Bitcoin, I am sure Bitcoin won't lose his value.
The only way that Bitcoin can't recover when it has no value anymore. But it is very unlikely to happen if we consider how the adoption of Bitcoin nowadays. Moreover, if there are more countries to adopt bitcoin seriously like El Salvador, Bitcoin probably increases its value. Since it is getting popular, we have more and more demands of Bitcoin. The increase of the demand is a positive sign for its future. So, I believe Bitcoin price will gradually to increase, it won't fall again below $20k.
I think that if it comes to 1$ then people demand will be higher as people select bitcoin at that time when price is not much higher so increasing demand will increase the Bitcoin's rate.
I doubt there will be high demand if Bitcoin falls to $1.
Most people must feel panic to see such low price, so people may lose their interest in Bitcoin. I really don't expect the price to fall $1 because I think it will be a disaster if it really happens.