The question is how much of that volume is the same money going back and forth. Taking sizable amounts of money out of the system is different. See 2010 flash crash".[1] There may be less real liquidity than transaction volume indicates.
The majority of the global volume is crypto-crypto, so I agree it's a valid concern, but the volume of fiat trades has gone way up over the past several months. For example, Coinbase added 1 million accounts in June - the vast majority by necessity are putting fiat in vs cashing out.
One flash crash happened on the ETH-USD pair on GDAX like a month ago, when a large dump triggered cascading margin calls. Bitcoin has had a few of those over the years too.
The volume on many exchanges are fake. It's pumped up using bots buying and selling among each other. Large dumping tends to go through dark pools or OTC. The real volumes are much lower.
[1] https://en.wikipedia.org/wiki/2010_Flash_Crash