Which problems does the average user get (except of a bit more filled disk space) when he has init scripts installed which are not used? (We are not talking about having two init systems installed, just the init scripts.)
"Doesn't make sense" is not an argument I think to prohibit something. "Does harm" is one, in my opinion.
There's not a practical way for any distro to handle this extreme edge case. As I mentioned before, the reason they conflict is because of conflicting dependencies on openrc and runit. Runit initscripts absolutely need to have runit as a dependency and likewise openrc initscripts absolutely need to have openrc as a dependency. The point of a package manager is to handle all of the dependency junk for you. It would make no sense to distribute those initscripts without the proper dependencies because they would be completely useless without openrc or runit installed.
Now as you said earlier, one could create "dummy packages" for this that strips out the init dependency to get around the conflict. That's fine, but that's clearly something a user would have to do on his/her own. You can't expect artix to literally duplicate all of their init packages, make a dummy version and put them in the repo. It would be very confusing for anyone doing a search for something like dhcpcd with pacman and seeing odd dummy packages popping up and the benefit would be almost nil.
If you really want to switch init systems on the fly for whatever reason, just write a script for it. It's really not that complicated to do. You just need to find all the packages with -openrc or -runit in their name and replace with them with the opposite. That's the most practical, sane solution to this "problem."