On Sat, Mar 21, 2020 at 11:40:24AM -0700, Dave Scotese via bitcoin-dev wrote:
> [Imagine] we also see mining power dropping off at a rate that
> suggests the few days [until retarget] might become a few weeks, and
> then, possibly, a few months or even the unthinkable, a few eons.  I'm
> curious to know if anyone has ideas on how this might be handled

There are only two practical solutions I'm aware of:

1. Do nothing
2. Hard fork a difficulty reduction

If bitcoins retain even a small fraction of their value compared to the
previous retarget period and if most mining equipment is still available
for operation, then doing nothing is probably the best choice---as block
space becomes scarcer, transaction feerates will increase and miners
will be incentivized to increase their block production rate.

If the bitcoin price has plummeted more than, say, 99% in two weeks
with no hope of short-term recovery or if a large fraction of mining
equipment has become unusable (again, say, 99% in two weeks with no
hope of short-term recovery), then it's probably worth Bitcoin users
discussing a hard fork to reduce difficulty to a currently sustainable
level.

-Dave