What other engineering discipline would say "there's no way to improve our reliability except stopping work altogether"? There's always a way to improve reliability. Arguably, with formal verification, you could ensure large parts of your system are perfectly reliable given simple assumptions.
The problem isn't that it's impossible -- it's that it's more expensive than just hiring one more engineer to keep papering over the problems.
What other engineering discipline would say "there's no way to improve our reliability except stopping work altogether"?
All of them. Are your roads more reliable when they're constantly being changed or when they are just being maintained? Is NASA achieving its reliability by constantly changing the designs of their ships, or by reusing the same design over and over?
Arguably, with formal verification, you could ensure large parts of your system are perfectly reliable given simple assumptions.
Yes. But a fixed formally verified system will still be more reliable than a formally verified system being constantly changed.
What was said wasn't that FB couldn't be more reliable. It's that they are already so reliable that only new changes introduce problems. Sure you can still work on minimizing those problems, but that's a different point.
The problem isn't that it's impossible -- it's that it's more expensive than just hiring one more engineer to keep papering over the problems.