Sideloading is surely good enough
In the aftermath of Sweeney's post, Microsoft's Phil Spencer made a tweet claiming that UWP is a "fully open ecosystem" that can be supported by "any store."
While many would probably quibble about calling Windows "open," this does seem to be technically true. Windows contains a big switch to control how it uses UWP apps with three settings. The strictest setting, which was the default when Windows 10 originally shipped, will only let you install apps from the store. The middle setting allows you to sideload UWP apps from anywhere, meaning that it can plausibly be used to enable apps from outside Microsoft's store. The apps must still have a digital signature, but they can be signed with any certificate that the system trusts. The third option is developer mode; this allows not just sideloading, but debugging and other developer-oriented capabilities.
This sideload setting is something we've explicitly advocated for, and we're glad to see it added to Windows 10. The Windows 10 November Update, version 1511, went a step further and made this setting the default. The PC as a platform retains its full power, and the user is in full control of what can and can't run, but by default the system is reasonably safe. With this option enabled, direct downloads, third-party stores, and third-party sales are all possible, enabling a putative UWP-compatible Steam, say, that signed all its games with its own signature.
It would be straightforward enough to make this work for apps downloaded from the Web, too. Just make the app's installer add the relevant certificate to the system.
This gives third parties the necessary tools to do what Sweeney wants. And there's certainly a role for third-party stores. Microsoft's rules go beyond the merely technical. They also prohibit certain kinds of adult content, for example, and one can readily imagine third-party stores without "blue laws." The ability to have stores without the same content restrictions, or with different purchasing mechanisms, or which offer their own friend and achievement systems, would make UWP more desirable to more developers.