Oracle is capable of changing the terms of the CDDL though (the CDDL has an auto-update clause), which would effectively allow for it to be replaced with a GPL-compatible version and it would affect all code put under that license (no matter who the copyright holder is). This is the same process that they used to switch from CDDLv1 to CDDLv2 for all their projects (though the only change was who had the right to change the license terms -- from Sun to Oracle).

But it's unlikely they'll do it. In recent years they've relicensed-under-GPL some things (like the DTrace userspace utilities) but I have doubts they'd ever do it for something like ZFS.