Altcoins have already lost a ton of BTC value, most of them have dropped around 25% against btc the last few days. The reason for this I think is that people look at things in dollar and start selling when they lose their precious fiat value.

There is no reason I can see for an altcoin to lose BTC value if BTC is losing USD value apart from people selling when they lose USD.
In a bear market the best thing to hold is BTC, it always goes down the least.