I see this as a weakness of the Unix security model, not a benefit of the walled garden. Personally I’d love a future where apps on my laptop are sandboxed just as thoroughly as iOS apps. I want to be able to install and run desktop software from the internet without worrying about it exfiltrating or cryptolockering my data. Running an application should be just as secure as visiting a website.
And that capability seems orthogonal to the App Store. I don’t see any reason we need a monopolistic app store on our phones to get good security. You don’t need anyone’s permission to make a website. Why can’t applications work the same way?