A lot of what a custom harness does isn't necessarily interacting with a model, it's the framework around it. Part of harness engineering is figuring out what can be done deterministically to avoid calling an LLM. A generational jump doesn't obsolete those areas, it just means you get the right answer (maybe) faster and easier when you do call the model.
There's a difference between their harness and your harness. The latter can be tuned to your preferences, while the former cannot. A custom harness can do everything that a packaged harness cannot. Good luck getting Claude Code to coordinate with multiple model providers or deeply integrate with your environment.
The point you're making is to be a consumer, which admittedly, is probably best for most people, but it's not going to satisfy the hacker. Normies aren't building custom harnesses, so your argument is DOA for the target demographic.
Also, vendor lock-in. No thanks.
We might have different views on where the SE as a profession is headed, I think. Again, I might be wrong, nobody knows the future, but we didn’t even use the word “harness” that much a year ago. There’s a decent chance in two years the harness itself as a concept might be obsolete. Depended on how fast your teams move, investing in a custom harness might be a bad resource allocation. Unless, obviously you’re in top tier companies which can benefit from it and pivot fast.
Again, this is all subjective risk assessment. I very much support any engineer to learn how to make harnesses, and I, myself, have a long way to go. We’ll see where we are in a year or two, I guess.