Most people are not 100% invested in btc. Rather they have multiple investments in both fiat and altcoin sectors. A total collapse would not effect them (as much) due to not having all their eggs in one basket.
Fiat is a safe haven at the moment.
The only trouble is (in my view) that if it keeps going like this, people will start to worry about the exchanges themselves. They'll want their fiat in real bank accounts - not in their trading accounts.