I'm no expert on cryptocurriencies but the only ALT Coin that would probably survive a crash like that will be TETHER. Tether was designed to be always pegged around a dollar $1, so even if BTC crashed like in your hypothetical scenario or skyrocketed back to $20000, Tether's market price will always be(or tries to) be around $1.
If we talk about the worst case scenario then to me it will be the collapse of crypto market.
First of all it is almost impossible that bitcoin will hit zero or 1$ and why it is impossible to hit bitcoin zero because following must be happened.
All the countries joins together to enforce the ban on bitcoin.
All the countries start to crack the bitcoin holders.
Bitcoin develop a bug that its developer unable to solve.
If this scenario is developed then what will happened.
Bitcoin holders will lose all their investment.
Crypto currencies will lost their complete trust and as a result all the currencies will become to zero.
I'm no expert on cryptocurriencies but the only ALT Coin that would probably survive a crash like that will be TETHER. Tether was designed to be always pegged around a dollar $1, so even if BTC crashed like in your hypothetical scenario or skyrocketed back to $20000, Tether's market price will always be(or tries to) be around $1.
Those would have to be coins that either can stand on their own, or have a completely independent community.
For me I would say....
Ethereum
EOS
Steem
If Bitcoin was to crash to a dollar, which is incredibly unlikely, I think we would see the death of something like 99% of all of the other coins.