I agree with your sentiments, but you’re really objecting to walled gardens (a term you also use), not ecosystems. Ecosystems are good things; they grow up organically around successful systems and make it possible to extend and adapt those systems into new places and new purposes.

What iOS has isn’t an ecosystem, it’s a hothouse.

I’d say the reason we have such clean “separation of church and state” on (primarily) Windows PCs is because Windows has such a robust ecosystem. That’s really Windows’ signature feature.