What expert would build a cli tool to avoid writing a bash one-liner?

I cannot judge without more context. Depending on the one liner, the problem at hand, and overall situation that can be justified. A CLI is straightforward to create.

A bash oneliner can be a chain of 5+ programs, each buffering the stdin/out, what if the CLI is doing the same operation via streams instead? Just a random example but that can easily be worth it

Sure, then there's a point where the extra taste isn't worth its cost.

I just don't like the fictional straw man where an expert has somehow been brainwashed by AI into forgetting everything they ever knew.

Wait... Did you just fictional strawman a fictional strawman?

Because I don't think I've ever heard anyone say an expert gets brainwashed by AI into forgetting everything they ever knew. Maybe losing some skills that require regular exercise. Or get lazy about implementations when spawning dozens of super useful agents. But come on. Don't build your strawman out of a fictional strawman.

Perhaps the ones that were experts before letting their brain rotten by AI.

Bash is one of the most complicated languages in common use and is horribly error-prone. It's almost never useful alone, but as an interface to call other CLI tools. I don't think this is a particularly useful comparison.

Same way an excel spreadsheet can be more expensive to maintain than a web app.