I think in time people will realize harness is essentially a more complicated .vimrc or .zshrc;

And yes, you can install gigantic plugins in those places - e.g. Codex; but the point is everyone will have exactly what they have customized towards. The more atomic a building block is, the easier it can be adapted into any kind of configuration.

I think the pain of selling a harness is if your target market understand what a harness is, then they can build it to exactly how they'd like it without much effort. If they don't, then the harness wouldn't be very useful to them in the first place.