There might still be some advantages of ZFS over btrfs, but I haven’t used either one at all, so I can’t really be sure.

Curently, there are none. In fact, BTRFS has outperformed ZFS in every aspect in the past few years, including filesystem growth (when changing drives, put in bigger ones, something you could never do with ZFS).

Outside of that, the BSDs are basically just different distros. Back in the 90s, when there was a lot more diversity in Unix, a lot of people just started out with *BSD because there was no clear choice at the time. People just like to use what they are more comfortable with - but most new users pick Linux over BSD these days, and a lot of people who started out on BSD have assimilated onto Linux.

The main reason is more drivers and software. Sure, it might be fun compiling from source when you’re young, but at the end of the day, when you wanna get work done, you really can’t tell your customer (or boss) “look, I really can’t deal with this right now, I’m building FF from source”. Also, one of the main reasons why Gentoo and LFS have a fairly small user base.

Still, diversity is a good, nice thing, especially with the advent of systemd. So I’m glad we still have the BSDs around, even if I disagree with their stance toward the GPL.

There are distros that don’t use systemd, Void being the most prominent of them all (mainly because of the number of packages it has in it’s repo).