Discover is very configurable wrt the backends it can have. However, it does not provide for the case where none of the type 'application backend' is available, not through missing dependencies, but choice at build time. Discover then still has its use for Application Addons, Plasma Addons, and as fwupd frontend, but the displayed 'welcome' page in serious-warning style is not helpful to the distribution that may never get an interface to its package management, or the user who consciously built Discover like that. It should be possible to flip a switch e.g. WITHOUT_APPLICATION_BACKEND at build time that makes it clear that lack thereof does not represent a packaging oversight or missing dependency.
I'm not sure, this is not what Discover is for. Discover is to make the resources to enhance the OS available to the user. Using discover without it is more an artefact of how it's implemented more than an actual feature.
You're talking about Gentoo, right? If so, I know there's no PackageKit backend, but the user could feasibly install the Snap or Flatpak backend, right? And the distro could package Discover with one of them.
In case of Gentoo building the flatpak backend should fix this.
That's a workaround for an artificial problem, isn't it? And it assumes I want every user on my system to be able to install flatpak images (or any packages, for that matter).
What's the use case for intentionally using Discover with no app backends? GUI add-on and firmware updates?
What I'm getting at is that the kind of user who prefers to use the command line for package management is the kind of user who should use the command-line to manage their firmware updates too. I don't know if there's a CLI tool for managing KNS content, but if there is, said user should just use that. The kind of user who wants a nice GUI experience for updating and installing content for everything *except* for software is a very rare user. I get that perhaps you are one such user. But it is such a rare use case that if you want this done, you'll probably have to submit the code yourself. :) Personally I would be willing to consider such a merge request, but absent that, I don't know if it's worth keeping this bug report open, sorry. :)
On the topic of knewstuff-tool (which is the sort of... semi-planned name for it), there isn't yet, but, as that paranthesis suggests, building one is kind of planned :)