If PetroChina were to lose 50% of its value, the stock market would hurt. There would be cascading effects across the markets and those effects would be fairly drastic. However it would not be realistic to assume that General Electric would also lose 50% of its value because of PetroChina's crash.
If Bitcoin were to lose 50% of its value tomorrow, we could reasonably expect every other cryptocurrency to also drop by an average of 50% (Except for Tether). We would expect this drop in the small altcoins as well as more major altcoins like Cardano and Ethereum, even if the reason for Bitcoin's crash had nothing to do with the other coins that are affected.
The reasons for that are debatable. I'm not saying that everyone has a lack of knowledge, but personally I see enough differentiation between altcoins that a movement in BTC would not cause me to sell my ETH or vice-versa. Maybe that just leaves me as a bag holder ;)