That's not an answer to my question. For example, I see no evidence Debian is moving toward app images as the standard way to distribute software. Same goes with Arch and I'm sure many others.

And btw SteamOS is absolutely not a sandboxed environment. It just has a read-only OS filesystem so they can safely blow it away upon upgrade.

Debian and Arch are exceptions. I believe maintainers of every other mainstream distro are exploring immutable distros or at least shipping confined apps.