Are Flatpaks gaining popularity?

Dogmas are always bad!

Sandboxed apps do have a general advantage over non sandboxed ones for the public and generally new Linux users, as Ubuntu folks quickly discovered, and made it their policy (Firmenpolitik). No need to mess with firetools, the command-line, etc. - Par example.

An idea that @pebcak mentioned was, that Arch Linux be not an immutable system.

But as @Shjim said, your system, your choice!

I can go for that.