I agree, sandboxing is a must-have feature. xdg-app has a system of runtime so it can run on any distro as long as you have installed the runtime needed by the app. Also GNOME and Papyros will have it while KDE seems interested in it also so it should have quite a lot of support.
xdg-app looks good, but I think we're still at least a couple of years away from the point where a developer can use xdg-app to reach the majority of desktop Linux users. That's a long time in software development terms.
You're surely right, but to me the important aspect is not only to make a package that run everywhere but also bring the mobile security scheme to desktop ie have apps that are sandboxed and run on top of a well-defined runtime with well-defined APIs.
In that sense Xdg-Apps have a bigger mission that AppImage so maybe we can't really compare them
