They did that. It's called "btrfs".

A stable clean-room ZFS with on-disk compatibility would be a huge task. How long did stable NTFS write capability take? And NTFS is a much simpler filesystem. It would also be a huge waste of time given that btrfs and bcachefs exist, and that ZFS is fine to use license-wise – it's just distribution that's a tad awkward (but not overly so).

Interesting to note here that btrfs came from Oracle.

"Chris Mason is the founding developer of btrfs, which he began working on in 2007 while working at Oracle. This leads many people to believe that btrfs is an Oracle project—it is not. The project belonged to Mason, not to his employer, and it remains a community project unencumbered by corporate ownership to this day."

https://arstechnica.com/gadgets/2021/09/examining-btrfs-linu...

That's countered by btrfs's source code always beginning with "Copyright (C) 2007 Oracle."

Maybe it was Mason's pet project within the company, but there is no ambiguity that Oracle owns it. It is an Oracle project.

A copyright line doesn't make it an "Oracle project". That implies a high level of control/involvement in the project.

It shows belonging, at the very least. The quote from Jim Salter was "The project belonged to Mason, not to his employer" (emphasis added). The copyright line demonstrably and incontestably refutes this claim. btrfs belongs to Oracle.

I don't think that would work; all of the changes since the fork are also CDDL and they aren't owned by any one entity/person. (IANAL)

Would you not basically be starting over at that point, though?