If BTC price dips that much in such a short period of time, it will be unusable for that exact same reason, not because of mining or anything else, every bear market that bitcoin had took months to actually hit 85% drop max , it took 400 days in 2013-2014 and 363 days in 2018 only to hit an average of 85% drop, and then it takes months or sometimes even years before people start trusting bitcoin again, some actually leave for good and never come back.

but with all honesty if bitcoin falls to 750$  tomorrow or even in a course of say 2-3 months (90% drop from where we at right now) , it's very likely that bitcoin will be considered unusable by most investors out there, and there will be no buyers to lift price back up.

with that being said, I see no reason why does 2016 blocks adjustment can't be changed, newer difficulty adjustments algorithms have been implemented and tested on other coins, they might work just fine on bitcoin , but this is no where near top priority now, scaling layer 2 seems to be the only thing that matters now, and when the time comes , i do expect a tweak in the difficulty adjustment protocol.