by pcalvert » So if I install Debian and then reconfigure it the way I want, it's not Debian anymore?
No, that should be obvious. Debian is open source, and not only can you configure
it any way you want, you also can modify it, etc.
But if , after configuring, modifying, adding to or taking away, what ever, when
the persons start calling it another distro, and distributing it ,using a new name, then it is no longer Debian,...this applies to Devuan, Ubunto, Linux Mint, etc,.... All are based on Debian yes, but they have been changed significantly, and the developers have chosen to use another name, thus creating another distro.
If you are using Debian, and configure it to suit your needs, it is still Debian.
When you start mixing in packages ,etc that are from other distros, it is no longer Debian, but a mix. known as a "frankendebian",... https://wiki.debian.org/DontBreakDebian
There is nothing wrong with developing another distro , for what ever reasons,
in this case , the people that are developing Devuan , did not like the idea of Debian using systemd, they think it is a mistake, so they started what is called a "fork", and now it is another distro, based on Debian, but without systemd.
Personally I don't think the developers of Debian just accidentally started using systemd instead of systemV , by mistake, I think there was a lot of planning involved, and they moved into systemd intentionally, not by mistake.