if mining capacity drops sharply, confirmation may takes forever and kill bitcoin
That isn't the way the bitcoin protocol works. It will adjust to a 10 minute confirmation regardless of the direction difficulty goes.
If network hashrate drops by a small amount - then what you say is correct, if hashrate drops by 90% in an extremely short period of time, it'll take 140 days to confirm all the necessary blocks for the next diff adjustment.
I'm not going to lose sleep over that scenario.