As to "bitcoin is not the absolute monopoly", this is true, but the market is REMARKABLY synchronized. It is as if the whole of crypto was essentially a single token. Bitcoin has remained around 33-35% of market cap during all this falling of 60% or more, give or take a few percent. So there seems to be one single "crypto" thing out there for greater fools.
Some coins hardly budged when Bitcoin dropped - Monero for example. Presumably because the people who were buying it were buying to spend in the dark markets.