We did devalue the dollar against the gold standard significantly in 1934. Not entirely unlike a "fork", since it's a declaration that, whoops, your dollars are now worth 40% less gold than you thought.

In 1931 Britain floated the pound, which arguably the US ought to have done too. Suddenly, your paper is no longer convertible to gold at a fixed rate. That's an even more dramatic change than the Ethereum fork.