I agree with you, and I cringed a little when I read the following from the OP:

> There are some fundamental reasons why conversational 3rd party platforms are hard.

In my mind the big fundamental problem here is the "3rd party". I'd love to have an "AI assistant" or an "AI buddy" that could watch everything I do and say and write and really get to know me super well... as long as I can be confident that I own and control everything it observes and learns. I sure as hell don't want a 3rd party involved! But alas, I don't see a way we get there that doesn't involve Amazon or Meta or Google or OpenAI sitting between me and my "AI" tools, at least in the short run.