If you're not willing to do full system upgrades on the regular, why are you running a rolling-release distro? That's literally the whole point.

Honestly, running pacman -Syu on an Arch machine that sat untouched for four years was a much better experience than any of the Ubuntu and Debian dist-upgrades I’ve had to suffer through. I did have to go and merge the .pacnews, but that’s it. (Granted, for Ubuntu the last one was something like 9.10 to 10.04, things might have improved since then.) So having an infrequently-updated machine is entirely feasible, as long as you don’t try partial upgrades. (Except that one time when the StrongSwan upstream decided it was a good idea to rename their units in such a way that an old configuration with ended up running the wrong IPsec daemon after an upgrade. That was a frustrating couple of hours.)

I plan on abandoning this experiment as soon as I have the time and interest.

In my comment above I mentioned this was a spare laptop, my primary laptop which ran debian abruptly failed pressing this thing into regular daily use, so I used the Arch install I originally put on it for an egpu experiment.

The thing is, it’s not that this problem can be fixed by running pacman -Syu, it is that, in half a dozen years of running Arch on several machines, the only way I could get into this particular failure state was when I ignored every bit of documentation in the name of laziness and did pacman -S thing instead of pacman -Su thing. (Or when I built things from outside the official repos and failed to keep them up to date, but I’m going to guess you’re not running a custom build of Clang, because that is the kind of pain you don’t forget.) Theoretically you might have caught a short window of inconsistent state on a package mirror, but again, from my experience, it’s always just a partial upgrade I did with my own hands. (Compared to apt, pacman is awesome, but its willingness to let you do stupid things could use some adjustment.)

On to more constructive advice—if your latest update was in the last couple of weeks, just

  pacman -Su

may work to pull your machine forward to a consistent state corresponding to the most recent installed package. If not, you can use the <https://wiki.archlinux.org/title/Arch_Linux_Archive>: get the last update date by an incantation such as

  pacman -Qi | sed -n 's/Install Date *: //p' | xargs -d '\n' -n 1 date -I -d | sort -nu | tail -1

(which I just cooked up, so there are surely better ones, or just look at the tail of /var/log/pacman.log), temporarily replace your /etc/pacman.d/mirrorlist with

  Server=https://archive.archlinux.org/repos/YYYY/MM/DD/$repo/os/$arch

and run

  pacman -Syu # sic!

Either way, you may see a little bit of breakage (though, in my experience, it’s unlikely), but nothing you wouldn’t have had to deal with when you properly installed your current set of packages in the first place.