I seem to recall that Sun put ZFS under a license that has been deemed incompatible with the GPL by pretty much everyone except Canonical, and so that it can't be integrated with the kernel but must remain a "bolt-on" for legal reasons.
It won't surprise me if one of these days Oracle decides that Canonical is a ripe lawsuit target over ZFS.
Why would they if they haven't already? It's not like Canonical has a lot of money. But, yes, anyone else who Oracle might actually elect to sue won't go anywhere near ZFS.
Btrfs has also been improving. Certainly not for all use cases but it's the default for desktop Fedora now and some Synology NASs use it, among others.
On one hand ZFS has held down btrfs because "why bother" when you already have something that does much of what you want, but on the other hand it shows what can be done.
Yeah, but even if bolt it on (which Ubuntu does for you, any other distro it's easy to do) it's still a "bolt-on" that doesn't integrate as fully as it could if it were the native filesystem, allowing full system rollback, snapshotting, boot management, etc.
It's similar to the "polish" available on macOS or Windows, Linux has all the pieces but you have to assemble them yourselves, even today.
This would be a more convincing argument if your facts weren't over half a decade out of date. DTrace for Linux was relicensed under the UPL (with kernel components under GPLv2, just like the kernel) way back in 2017.
(speaking as the guy who wrote the scripts to change all the license text, though the actual relicensing was due to a lot of work on my boss's part.)
I think the Flying Circus in Bealeton, VA (near DC) brought back free wing-walking (i.e. not strapped to the plane), and continues their barnstorming airshows to this day:
https://en.wikipedia.org/wiki/Electronika_BK
Fun to ponder what would it be like if the PDP-11 (and descendents like VAX) architecture had become dominant instead of x86.