It doesn't matter if you don't call it bitcoin, if it is possible to make transactions it's a currency.
Too bad if bitcoin were to lose that ability. What happens when miners leave in a rush is what you saw with namecoin back before the merge.
Only this time you need an asic to even make those baby steps to the next adjustment during your lifetime.
You speak of miners as if they are a single borg-like entity. If some miners abandon ship to a unprofitable doomed fork then the miners which remain will reap larger profits. If enough miners leave them it will leave a vacuum and new miners will deploy hardware into the gap.
While it is true Bitcoin difficulty lags hashing power under most scenarios it is a non-issue. The time between difficulty adjustments is 2016 blocks or ~14 days under normal conditions. Say 50% of miners leave right after a difficulty adjustment (worst possible time) then the next difficulty adjustment takes one month. Transacitons take twice as long, it is annoying but hardly earth shattering and in a month the network adjusts.
Lets take an even more unrealistic scenario where 75% of miners leave (once again right after difficulty change) it will take about two months to adjust and network will be back to normal.
Sure if 99% of miners left it would cripple the network but miners are highly fragmented the idea that 99% of miners would agree to anything is laughable.