It should be noted that a) FreeBSD is doing work to keep software working without SystemD (an obvious point, I know, but it might not be obvious that this means that work also benefits Debian) - so long as we don't see any crazy dependency on SystemD from Apt or the installer itself (hopefully unlikely) - there shouldn't be any issues. b) Debian/GNU Linux still supports more than one init-system[2] -- at least for now.
While the long fingers of SystemD reaches for more and more of Linux, as long as developers are supporting other systems at all, there should be a way forward for distributions that doesn't want to incorporate SystemD. So this includes stuff like native Windows support, building on OS X, and packaging for the various BSDs -- all that work that goes into the big packages should continue to benefit Linux users that want to avoid SystemD for one reason or another.
> I was excited to try Debian/kFreeBSD around 2012ish? but it was kind of painful -- a lot of important FreeBSD userland tools weren't available in packages
As far as I know, this should've improved quite a bit. The biggest bar for entry (last I checked) was that the nightly installer builds might not work out of the box (one possible workaround would be to install Wheezy/7.0 and then dist-upgrade).
To be clear, I've yet to play with Debian/kFreeBSD, so this mostly from impressions from the list (and I'm not quite caught up with the latest). AFAIK the installer should now support installing on zfs rootfs, for example.
[1] https://lists.debian.org/debian-bsd/
[2] https://wiki.debian.org/systemd#Installing_without_systemd
As a FreeBSD user and a Debian user, I was excited to try Debian/kFreeBSD around 2012ish? but it was kind of painful -- a lot of important FreeBSD userland tools weren't available in packages, so you had to do a lot of messing about to get things you might need. It might be interesting to run a Debian/kFreeBSD jail inside a regular FreeBSD install, but at that point, you may as well just bite the bullet and do things in regular FreeBSD.