Its honestly not that bad installing apps on these distros. Yeah the main one is flatpak, secondly appimages as easy as ever, and any other app can be installed via distrobox and exported if needed. If not that, then apps can also be layered onto the system if needed.
Ultimately it boils down to user preferences, but considering the growth and popularity of Immutable versions of distros, I would be surprised to not see a few more attempts from the Arch side of Linux.