One thing that makes easier to switch with LLMs as opposed to other tech is the text-in text-out nature of the LLMs. All specs and data are in text files mostly in natural language which is patent free. This makes it harder to lock in content.

As you have written, the agentic harness is a slightly different matter with plugins and features. But the core tech makes replicating software a lot easier so you can in principle use an agent to make it easier to switch from its own harness. There are caveats to this since in practice the quality and feature sets of harnesses are not the same. But in my opinion, it is far easier to switch agents than switching other tech that came before them that used proprietary formats and apis to lock you in.