More to the point, it's a field where we're constantly told that our experiences from a month ago are in no way relevant and that the latest thing is fundamentally different to what we know. Should expertise degrade just as quickly?

Yes. The worst company I worked for was the one that allowed the guy who was a programming expert from like 30 years ago to make all important decisions.

Don't take it wrong way, but it's software. It's not that deep for 99% of cases.