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,
You said it. Some altcoins were survived when BTC downtrend goes on, but it just less than 10% of total altcoins were survived. I don't know why people sell their altcoins to buy BTC when the BTC price goes down. It will be good if they sell their altcoins to FIAT or Stablecoins directly.