Don't forget that transaction fees help to entice mining too, not just the block reward. The death spiral never happened during the frequent difficulty swaps with BCH because fees simply went up until it was profitable to mine a block -- to as much as 40% of the block reward. There's no reason in principle it couldn't go much higher if necessary.
There's a limit to how high they can go (can't offer more in transaction fees than you actually have) and even before you hit that point you may encounter the back side of Bitcoin's Laffer Curve where the expense results in lower total transaction fees per block even as individual fees rise.
It's possible that point would be far beyond any conceivable price crash, though, I don't know how to put it into anything resembling concrete numbers.
It wouldn't need to be all of the ASIC mining, of course, just a large enough portion of it to result in massively increased block times.