>

I have worked in QA for over a decade and I fully support this answer. With more users/volume comes more edge case usage.
Not surprised. That’s why you use a test environment before implementation. On something as big as the FAA probably roll it out in parts too. >

That's not fair. They had a test system.
They just didn't have a separate production system. >

In every company I've ever worked for I always ask if we have a clone of production for testing.
Only once have I been told "yes".
Every other time, I'm told that the QA environment is "close enough". >

Until it isn't.  But why spend money doing something useless until it costs you millions of dollars and/or customers? >

Why wouldn’t you just make a new copy of the production environment and overwrite the QA environment with it so you have a test system that is up to date to test your code? >

The biggest hurdle isn't the code, it's the volume of users that exist in Prod vs in sub-prod environments.
And just straight copying prod data down isn't a great idea, because it contains PII.