Do yourself a favor and use standard Debian. Honestly, I don't know why anyone bothers with Ubuntu anymore. It doesn't really add much over Debian except for it's shitty release cycle.
I think your wording was a bit too hostile, but I basically agree. If you yourself are comfortable with Debian, you can just install stable and ignore it for years in a way you just can't with Mint or Ubuntu. Every time I've tried it I've always just ended up switching them after the n-th time of finding out the automatic updates broke at some random point a year or so ago and now there are weird problems with the sources and things not parsing correctly when I try to manually update.
Debian stable with xfce is a good enough XP-ish interface that no one really gets confused by it, and is stable enough that you can just forget about it for like forever and everything is still fine.
Ubuntu has non-free software enabled out of the box unlike Debian. To me this is a negative, but to some it could be the difference between their wifi working or not. It's not that this is hard to change, but if you're already planning to make changes right away, maybe it's not the best distro for your parents. I've taken a liking to some pretty niche distros in the last few years. GuixSD and Void in particular. I used Arch for a long time as well. I love these distros, but I'm not sure they're right for my parents. Maybe if I learned how to make a customized installer that would ship my configs and certain software out of the box I could go that route.
Eh, I think generally if you want to set up linux for a non-technical person, regardless you're going to need to personally spend a couple hours configuring and setting everything up for them initially, so the difference of editing sources.list and adding "main contrib non-free" to a couple lines is pretty minimal.
It is a distribution that actually cares about the out of the box desktop experience, with the right set of drivers available by default regardless of FOSS religion.
Debian has a rolling release. Very easy to understand.
Ubuntu has 6-month releases and LTS releases. In my experience, upgrading Ubuntu often fails and requires manual intervention whereas Debian upgrades just work.
If you get a few years behind on Ubuntu upgrades you either have to reinstall or apply each upgrade in sequence.
Debian Stable, which is the default, is not rolling. Debian has 2-year releases (Stable), a rolling release that occasionally freezes (Testing) and an actual rolling release (Unstable/Sid).
Personally I've been using Unstable on my laptop for over a decade without any real issues, but then again I keep a very lean desktop (LXDM + awesome), so there is little to go wrong compared to a full DE.
Do yourself a favor and use standard Debian. Honestly, I don't know why anyone bothers with Ubuntu anymore. It doesn't really add much over Debian except for it's shitty release cycle.