How has Snap failed? Snap lets you install apps like Spotify easily on linux and in a security sandbox. Also snap doesn't have the main issue listed in this article of every app needing filesystem access. Snap has the personal-file interface which you need explicit permission to upload a snap to the Snap Store that accesses this interface: https://snapcraft.io/docs/personal-files-interface .
Source: working on a open source snap applications (without the personal-file interface despite being able to install packages into the sandboxed $SNAP_USER_DATA directory): https://github.com/argosopentech/argos-translate
Flatpak has portals for the same purpose. Unfortunately, many apps don't support portals yet. The Flatpak for Spotify does not allow access to home, only to the music directory (read-only). So, it's running in a proper sandbox too.
Source: working on a open source snap applications (without the personal-file interface despite being able to install packages into the sandboxed $SNAP_USER_DATA directory): https://github.com/argosopentech/argos-translate