Not really. Anthropic for example sells both the harness and the models as a unified kit via Claude Code, it is in their best interest to make sure both parts work as well as possible, via reinforcement learning of previous usage as well for new model performance increases.
That's not true that anyone can write a good harness because the LLM providers have information like prompts that they can RL train off of that someone writing their own harness would not have. Therefore a good and proprietary harness is a moat.