The same argument has been made about monarchies; no rational king would ruin the kingdom for his son. A quick perusal of the history books makes a mockery of that theory.

Turns out humans aren’t perfectly rational, and handing a huge amount of control to a tiny number of rich people is a fantastic way to discover that while any given rich person might be smart and what not, there’s a non trivial chance that their heir is an idiot and will ruin whatever they’re given. This is entertaining for personal fortunes, but not so funny when they control a currency.