If bitcoin will hit $6000 then it will probably end up with $5800 or $5900 ethereum can hit $400 but that does not matter there will be bounce for me.
Normally cryptocurrencies price should not be correlated to each other but most of the time, it just follows the bitcoin price, and I agree with you if BTC will drop under $6000, ethereum price will drop under $400 unfortunately.
There was a time when bitcoin and altcoin were considered reciprocal of each other. When one moves up there must be downfall for other. But now, they are totally independent in that regard. But you cant consider them independent in every regard. Now, they are just lined up at back of bitcoin. Wherever bitcoin goes, they are following it. Up or down, they are making good moves now.