Why not use ZFS on Linux?
Depending on an out-of-tree module can get pretty crappy on Linux. The upstream kernel will break you a lot. If your distro is on top of keeping your out-of-tree feature working then great. It gets messy whenever that is not the case and/or suddenly you want a driver or feature only available in a newer kernel.
Larry can't change the fact that GPL is the source of incompatibility - the general interpretation is that GPLv2 forbids code that has stricter licensing terms than GPLv2, and CDDL happens to have that - in form of patent grant.
It's also why GPLv3 code has a license issue with Linux kernel, which is GPLv2.
There are probably other options, too.
It's a pipe dream anyway, of course.
Another is that all the porting work in ZoL that isn't already GPLv2 is CDDL - and Oracle is not the copyright owner. In fact, the copyright owners would not want to change the license to GPLv2, unless maybe it was done as dual license.
On the other hand, for a simple set-up on a lower resource system, EXT4 has lots of advantages.
[0] https://www.jodybruchon.com/2017/03/07/zfs-wont-save-you-fan...
And I haven't experienced file corruption in the 5+ years I've used ZFS. Of course, ZFS isn't a magic bullet that solves all ills, but I've been very happy with it. I still, of course, have multiple backups of important data, but now I'm more confident about the integrity of those backups.