I’d say that harnesses almost by definition are the parts that you want to keep customizable. That won’t get absorbed into the weights.

Depends on your product strategy. If you only care about how your model will be used in the context of a harness (perhaps, specifically the harness that you designed), then the incentive is plainly there to optimize the weights within the context of the harness.

What is being absorbed into weights is tool usage. It is incredibly counterproductive to train models on a specific harness, when instead it can be trained to reason about the tools that it has available to itself and how to best use those tools to accomplish it's goal.

Would you rather hire an engineer that can adopt to your org's prefered tooling, or hire an engineer that can only perform well with their own favorite tools? It's the same thing.

My naive intuition is that as harnesses converge on shape and models improve the first party advantage will mostly disappear.