> in the US we've long established an expectation that if you have funds in a bank account at a registered bank, the government would back those funds

Not since around 2010. The ~2007 crash was not without consequence.

Indeed. Deposits at IndyMac Bank were only paid 50 cents on the dollar in 2008.

Your funds are insured up to $250k. That's the expectation and reality. Every account has the disclaimer that amounts in excess of $250k are not insured.

You are right of course, but we've established a moral hazard

There will be a cost to suddenly altering expectations, I guess there's a chance we might see what that cost is now (but probably they will just get a boring bailout)

No we have not established a moral hazard. It's always been $250k. You are trying to make one up implying that we are "suddenly altering expectations". The narrative has always been, FDIC insured accounts are insured up to $250k.

I've known this since I opened my first bank account.

It's a moral hazard if people over 250K got bailed out in the past. Look up the Temporary Liquidity Guarantee Program.