Of course, as I said, I don't think this gonna happen. And when i say "disappears" is not like a magic trick but progressively losing his value.
I just was thinking that if this somehow happens, try to think about thousand of people trying to withdraw their money from Coinbase, etc due to the fear of lose their investments, I don't think any exchange can afford that.
That being said, if convert alcoins to fiat almost only way is convert to BT first, it would be a disaster. Even if ethereum get the spot from BT, as other user said, there is no trust anymore in cryptocurrencies.
I know this is kind of apocalyptic situation xD I don't want to make you feel unconfortable