I don't think all altcoins depend on BTC, the proof is that there is altcoins that can still survived if BTC goes down, but most altcoins follow the btc trend, maybe because BTC is so dominant in the crypto that many altcoin holders sell all their assets to buy BTC,
but few altcoins do that. mostly if there is a decrease in bitcoin, there will be a lot of altcoins that go down, especially if the altcoin is just a momentary fomo action, of course it will come back down also following bitcoin.