I don't use arch

Debian would not create and maintain a "core debian" variant just to be installed then receive the extra packages

Debian server minimal, is kind of a "core Debian". There are netinst versions that can be even smaller. The Debian base image for Docker is even smaller than all that.

There is also an Ubuntu minimal install that you could call "core Ubuntu".

But more importantly, and I can't stress this enough: YOU CAN SWITCH DISTROS WITHOUT REINSTALLING. Might need to do some cleanup afterwards, but it's perfectly doable, more so between Debian based ones.

How would you do that? Debian would not create and maintain a “core debian” variant just to be installed then receive the extra packages.

To be honest, I do not fully understand your question here. Could you rephrase?

Would the *ubuntu packages replace, instead of add on top of default debian packages?

They must not replace. If they are merely installing KDE on top of Ubuntu, then theres nothing to do here. The work is already done for us. But if it is doing more than taht, then they should be different packages building on top of the default debian packages for KDE et al.

Sort of like how LunarVim is a distribution of NeoVim. It is the same NeoVim, but with pre-configurations and plugins shipped OOTB, and it can be packaged separately.

What about dependencies?

Thats the beauty of this. Package managers are already equipped with dependency management. It is far easier to manage dependencies with a package rather than rolling out your own distribution. It is literally one of the biggest reasons why we use package managers to begin with. We dont want dependency hell!

Would debian have to coordinate with all *ubuntu maintainers (and they too, between them) for compatibility tests every time debian needed to update one of its packages? Or they’d just update and *ubuntu would have to scramble to release fixes for what had been broken?

This is a debian specific question, so I will try to answer more generally. It would just have to be done in the same way any package is maintained on that distribution. And this varies by distro; some distributions have different workflows for their package maintenance. The point is that we make use of these already defined workflows that have worked for decades and been iterated on. It is much easier to package than to create a new distribution.

Not to mention convenience; would you have to download debian, download *ubuntu, install debian, then your *ubuntu?

Instead of installing *ubuntu, you install Debian, then run one command: sudo apt install *ubuntu. I see these as nearly equivalent. Moreover, it could be made to be an option in the distribution's installer, sort of like EndeavourOS and Fedora do it.

Why not then package the “core debian”, with the tested component versions that work with the *ubuntu packages you’re downloading?

That can be what I mean with it being an option in the installer. But if you mean maintaining a whole separate distribution just for this, well ... you are maintaining an entirel separate distribution just for this ... instead of just maintaining a package.

If you're basing your distro on another distro, you'll need to modify your dependencies to fit the existing packages anyway. It seems like the only difference is which repo the additional packages are being fetched from.

I don't see how this is more difficult on the user. It is running a simple command, and for a GUI package manager it would be a single button click, just like you'd do it in a graphical installer. It would indeed be almost like a series of check boxes.

As a user, it is much easier to check a box than reinstall my entire OS

A "theme" with the ability to replace key packages with compromised versions!

You and OP probably are confusing "distribution" with "UI customization". Granted, lazy distros may by simply that, but not all of them.

They are of course very useful, please do not misunderstand my post. None of what I said downplays the usefulness of these efforts. I am merely suggesting that the method in which they distribute them is not efficient. Maintaining a whole different Linux distribution just to distribute a different DE configuration is overkill. It is much easier to maintain a package instead.

From the user's perspective, installing Ubuntu and doing "sudo apt install [pre-configured KDE package to your liking]" is effortless and virtually indistinguishable from just installign Kubuntu. You get the full support from Ubuntu, whereas a different distribution may not. You are not needlessly breaking away from Ubuntu.

Honestly, it could even be an install option, like Fedora and EndeavourOS do. Do you miss out on anything doing this vs an entire different distro? I dont think so.

Again, a changed DE is pretty drastic, but it does not warrant a different installation process of the whole OS or system. You should only need to take out the parts you need to, and from a user's perspective, it should be possible to make it as simple as running a command or making a choice.

Name a single popular distro that follows op's description that isn't a novelty/fad.