Well, there’ll always be a long tail. But if you start with the popular apps and frameworks first, you get the most bang for your buck.

It wouldn't surprise me however if sandboxing is only a good model for GUI not console app.

IIRC the Windows 10X approach will be to put all non-sandboxed apps in the exact same environment.

As mentioned above, Flatpak is already doing the two most popular toolkits. Programs just don't use toolkits to open files. So there is not a lot of "buck" here, and the tail is rather long.