I don't think Android is "dumber" or less capable than Windows. In many ways the application sandbox actually gives owners a lot more control over their devices than a less locked down OS would, allowing them to restrict what information installed applications are allowed to access.
But what I think you're concerned about (and I agree) is that the flip side of that is that giving device owners more control over their apps also gives the OS developers more control, and Google's interests are not always perfectly aligned with the device owner's. There's a much wider market for apps than there is for operating systems, so sometimes app developers' interests will actually be better aligned with the device owner's than the OS developer's interests are.
One possible saving grace here is AOSP. In theory you could have multiple competing AOSP-based desktop OSs, each catering to a slightly different set of users. This would be close to the ideal situation in my opinion. Either that or Chrome, Firefox, Edge, and Ladybird all evolve into full fledged OSs with WASM-based apps.
I see your point, I do. It seems like all external software is going in the SaaS direction, where the vendor is keeping all of the data, so they are available over an API. So there are genuinely solid cases for Chromebooks.
The issue is how much power this gives to the vendors. I think we should be able to survive a vendor going poof, taking all our data with them. Having a general computing platform capable of mixing files and privileges seems to me like the only way of keeping this capability.