If its a local-first app, that could be very good! Even if browser+WASM with local storage that is a step up from web apps.

I don't like companies hoovering all data.

How long before that's no longer the case?

Only way to be sure it will never change is to build it yourself and make it free for everyone forever!

Though even if you build it, someone might eventually make you an offer you cannot refuse.

You can always refuse offers. Plenty of people enjoy working long hours and answering endless customer support issues for no revenue. Well, there must be someone out there.