central theme is that "adding manpower to a late software project makes it later."
Some engineers are just slow. I don't think it's much correlated with experience. If your goal was never to be a super productive programmer, I don't think you'll ever become one.
This might also be why there is this spectre of agism in the industry. Everyone knows there's greybeards out there that would eat your lunch and get paid the big bucks, but at the same time older programmers have hard times interviewing. At that age most of the really great programmers have probably already been fished out of the pond.
I will say, my recent stint of interviews over the past 6 weeks or so has been pretty painful. Everyone seems to be going through 6+ interview stages. The reality, even then, I'm not sure they're getting a great signal:noise outcome. Some of the tests are for things that will never be used for those jobs on any given day.
I do miss my 20's when an interview was maybe an hour and a half on an afternoon, and an offer shortly after.
The math is not that simple. The output of a Junior engineer is much different from that of a Senior engineer. You can hire any number of Junior engineers you want, and they won't have the vision and seasoned experience of a Senior engineer for producing higher quality code with less technical debt.
Even among Senior engineers there are different grades of experience/competence/capability.