You're still thinking in terms of what APIs would be used for, rather than what local computation enables.

For example, I'd like an AI to read everything I have on screen, so that I can ask at any time "why is that? Explain!" without having to copy paste the data and provide the whole context to a Google-like app.

But without privacy guarantee (and I mean technical one, not a pinky promise to be broken when VC funding runs out) there's no way I'd feed everything into an AI.