They'd probably most readily split along systemd versus non-systemd lines.

Whatever gets the job done. Usually, that's Windows for me because that's what we work with and I don't have a choice at work.
I'm partial to Mint, and used to manage CentOS servers. They were rock solid.

Hate what RedHat did to CentOS though, that was a dick move.

Would the Linux kids split along package manager lines (.rpm, .deb, etc.)? Or along DE lines (Gnome, MATE, XFCE, etc.)?

I can't see anyone arguing about package managers. Desktop environments; maybe, but I think many distributions offer you a choice of those and it's quite easy to switch and so it's not really a distribution issue. If anyone starts telling me to use or not to use this or that leads me to think they've got too much time on their hands.

I split over LTS versus Rolling Release... I prefer rolling

Oh, I prefer LTS for the stability. But, this opinion is not set in stone.