Sorry to be a pedant but since it took me a second to get it at first: "all but gone" implies that they're definitely still here, I think something like "almost gone" or "almost out of use" would be the right phrasing.

all but is an expression meaning almost. See e.g. https://grammarist.com/usage/all-but/. It's one of the most confusing English expressions I've encountered.

'all but' is an idiomatic synonym for 'almost'.

https://idioms.thefreedictionary.com/all+but

I don't see how that's relevant. _They can and will still fail_

What happens then they do?

You have more than one independent system. A typical glass panel install will have a primary ADAHRS (air data attitude and heading reference system which contains a MEMS magnetometer, gyro, accelerator, pitot static pressure transducers, and processor) and a secondary ADAHRS both connected to the primary flight displays. Should one fail, you have a fallback for your PFD. As a backup, were you would normally have a vacuum gyro, you have a small glass panel (the garmin offering is the G5) which contains it's own built in ADAHRS and battery and will continue running even with the entire aircraft electrical system disabled.

Most of the GA fleet is still vacuum powered for backup.

Why do people keep parroting this? I know of exactly zero aircraft currently produced that are retaining the vacuum system and I've seen very few retrofits that retain the vacuum on the engine. The aircraft in operation with vacuum systems use them as primary systems but are moving away from it with each annual.