True and I understand the caution considering Oracle is involved which are an awful company to do deal with (and their takeover of Sun was a disaster).
But really, this is a concern for distros. Not for end users. Yet many of the Linux users I speak to are somehow worried about this. Most can't even describe the provisions of the GPL so I don't really know what that's about. Just something they picked up, I guess.
Licensing concerns that prevent distros from using ZFS will sooner or later also have adverse effects on end users. Actually those effects are already there: The constant need to adapt a large patchset to the current kernel, meaning updates are a hassle. The lack of packaging in distributions, meaning updates are a hassle. And the lack of integration and related tooling, meaning many features can not be used (like a/b boots from snapshots after updates) easily, and installers won't know about ZFS so you have to install manually.
None of this is a worry about being sued as an end user. But all of those are worries that you life will be harder with ZFS, and a lot harder as soon as the first lawsuits hit anyone, because all the current (small) efforts to keep it working will cease immediately.
That is due to licensing reasons, yes. It makes maintaining the codebase even more complicated because when the kernel module API changes (which it very frequently does) you cannot just adapt it to your needs, you have to work around all the new changes that are there in the new version.
You have things backward. Licensing has nothing to do with it. Changes to the kernel are unnecessary. Maintaining the code base is also simplified by supporting the various kernel versions the way that they are currently supported.
But really, this is a concern for distros. Not for end users. Yet many of the Linux users I speak to are somehow worried about this. Most can't even describe the provisions of the GPL so I don't really know what that's about. Just something they picked up, I guess.