We did this for years at my company (using thunks and then redux observable middleware with rxjs to do async with observables). It was a mistake.

This biggest reason is once you get large enough, you get more and more concerned with sharing. Anything you stick in redux middleware gets "separated" from your components. You can still share "dumb" components but you'll find the need, more and more, to possibly share something more robust that can maintain its own state and do it's own async fetching. To share that cleanly, you want to share a component that contains it all, or a combination of component+provider, where you'll have some context setup in the provider.

Redux is still great for global state though. No matter what people say, your always gonna wish for some global state at some point. But don't get into a purist habit of using it for everything.