BTC is the first created in cryptocurrency or the king.
So, it will affect the trust of people who would want to invest or already invested into bitcoin. They will be getting into panic sell zone and continue dump which is not just bitcoin, altcoin will also get the effect.
but, usually if btc goes down, most of altcoins goes up or just stay.
btc goes up, altcoin goes down.
when btc stable, that's altcoin time to go up.
it depends on the trust. Probably, altcoin who can survive with better support or feature bypass the btc one day. I don't know. Just my 2 satoshi.