This is not true. Liquidity in these markets is steadily increasing. It is easier now to sell large amounts of Bitcoin than it was two years ago, and it was easier then than two years prior.
Have you tried to sell millions of dollars worth of bitcoin to USD? As soon as there is a down day the exchanges shutdown and stop people from trying to withdraw. The money to cash out billions doesn't exist. All these headlines of founders being billionaires is false. $1.5 billion fake USD pumped crypto. Good luck to anyone holding millions of dollars trying to cash out when the market turns downward. This is adult musical chairs. There will be millions of bag holders and rich exchange criminals.
Moving the goalposts, are we? The question here was if "liquidity is drying up", which is the opposite of what's happening. While you probably couldn't move a billion, you never could and that's not a sign of anything drying up.
A million dollars perhaps used to be problematic, but is now a drop in the bucket on any exchange's daily volume. A normal bank transfer will suffice, but check with your bank if you aren't moving those amounts regularly. For bigger volumes there is a quite functional OTC market as well.
It's easier than ever to sell for other cryptocurrencies, including Tethers. In a fire, though, it's not the size of the trading floor that matters, but the size of the exits.
The liquidity is increasing because of the current gold rush. People who have no business getting into this kind of investment are buying in, thus increasing liquidity. I can no longer count the amount of acquintances who have bought 5-10K USD worth of BTC or Litecoin or Ripple or whatever, without having even a cursory understanding of the mechanics behind it.
These types of investors also panic easily, once the bank run starts, nobody will be buying. In fact, I bet most exchanges will simply close up shop.