Why do you think customization can only viably done via changing the code of the application itself.

I think there is room for closed source platforms that are built on top of using LLMs via some sort of API that it exposes. For example, iOS can be closed source and LLMs can develop apps for it to expand the capabilities of one's phone.

Allowing total customization by a business can allow them to mess up the app itself or make other mistakes. I don't think it's the best interface for allowing others to extend the app.