As they say, never put all your eggs in one basket, and this is exactly why they say things like that. Always diversify your investments, not only in crypto, but in other stuff like gold, silver and real estate, which are some of the best options for those who have savings to invest.
In theory, for Bitcoin to crash to an extent which would make a rich become poor due to it if he has everything in Bitcoin, almost every entity, every exchange, every service and company will need to dump their coins and immediately stop using Bitcoin. The blockchain should stop and there should be no transactions. And, none of that sounds possible to me. So that's definitely not happening.