What solution would you use instead of Flatpak for sandboxing and reducing the workload of maintainers providing packages to many distributions at the same time?
I'd rather have a maintainer spend time on actually maintaining software instead of packaging it. They can package as AppImage, tarball and Flatpak and I'm happy. You don't have to use Flatpak, you know? Linux is about choice. I have never used any software which was available exclusively as Flatpak.