I mean sure, there's always the always the possibility that a majority of miners ally together, forfeit millions of dollars of mining rewards, and likely cause all of their cryptocurrency holdings to plummet in value, all in order to double-spend a few of their own transactions. But if we're going to worry about the possibility of everyone involved becoming irrational, then I'm going to worry about the more realistic scenario of my bank doing something surprising with my money and causing me to go through legal hell, especially if my lawyer and the judge decide to be irrational too since we're comparing against a case of whole groups of people acting irrationally.

It's also worth mentioning that Bitcoin and Ethereum have never had a 51% double-spend attack. People do occasionally go through legal hell related to their bank/paypal/whatever accounts, or have problems sending money through their bank to who they want to send it to.