For the average person working in tech, what's their most valuable credential?

I suspect for most it's the most "impressive job they've had, with a decay function for how long ago that was".

I’d agree with that for the most part.  It also adheres to Newton’s First Law of Physics: a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force… if you’ve got the momentum keep it going, harder to get moving again after stopping.