I think the "half life of knowledge" misses so many parts of what makes a SW developer good. Sure, there are new languages, libraries and frameworks to learn and keep up with. But the fundamental parts of programming don't change - it is still sequences, loops, if-statements. Also, quite a large part is independent of the language - how to break the problem down into parts, thinking through the scenarios that can occur, good naming, consistency in the small and large.

Not to mention everything else that isn't programming per se - domain knowledge, talking to people, negotiating, in which order to develop things...

More details on what I think makes a good dev: https://henrikwarne.com/2014/06/30/what-makes-a-good-program... and about programmer knowledge: https://henrikwarne.com/2014/12/15/programmer-knowledge/