The market cap of BTC is much larger than the available liquidity of BTC. The "modernization" of the BTC market with things like options trading that help window losses also brings features like automated trading and triggers. The two largest holders of BTC, BlackRock and Strategy account for all available liquidity if they were to suddenly sell everything. But there are other whales with large enough positions that could kick off a cascading series of automatic trades and a collapse of liquidity. It only takes one large enough whale to kick off an avalanche of automatic trades to destroy this tulip market. These trigger settings are opaque to but are a risk to everyone who is betting that they will not trip.
Then there is the 1 MHW overhead on every single transaction which translates to roughly a $1 million per month electric bill (miners generally pay less for electricity than we do). Which requires that at least that much new cash keep coming into the system. If people only used BTC for illicit purposes to launder fiat currency transactions (which on net do not add fresh currency) the BTC ecosystem would slowly bleed out. New investors have to keep being added or the whole thing unravels eventually.
If mining becomes unprofitable for long enough and the hash rate drops, mining difficulty will drop by 75% every 2016 blocks. It could take months before mining again becomes profitable when the block reward falls behind the cost of the electricity to produce a block. Right now this is around $20,000/BTC. If people are trying to bail at a rate higher than 400 transactions per minute, the queue to be in a block will start getting longer and transaction times could grow to days, weeks, even months. We have already seen the backlog grow to as long as 560,000 unconfirmed transactions -- this is not a hypothetical scenario. If BTC is declining and the transaction timeframe becomes worrisome, more people will decide to sell and a this could lead to a bank run and whale triggers.
There are fundamental known structural weaknesses to BTC that have been ignored while the value went up and up, but are going to show themselves when the price is stagnant or falls for too long.