Somewhere along the line you learn it's better to write a couple lines of shell script than 20,000 lines of Java.

I totally understand where that's coming from. A guy doing the same tasks/processes for 15+ years will get less and less from additional experience.

However, he will not do exactly the same thing. For instance a 10+ years guy might realize that instead of investigating a topic for 3 workdays, he can ask the colleague from another team he worked with 2 years ago, and get the answer in 20 mins. His processes will change a lot, so his output can increase exponentially even. Of course it's limited by his intelligence, i.e. his ability to learn from his experience. And it's likely to happen in steps. So a quite smooth line for a few months, then an aha-moment where he bumps his output drastically, then another smooth line.

Define output. Once you get to a certain level of competence, you can easily produce lines of codes, so if that's your metric then the output plateaus after some years.

A developer can only type so much each day, after all.