That the developer himself finds it absolutely necessary to push new code outside the window for upcoming versions of the kernel is a pretty good indication.

It was nowhere close to be mature enough to be in the kernel. The developer is nowhere close to be mature enough to be involved in the kernel. It’s better for everybody if it is developed separately and maybe integrated again at a later stage when the file system and tooling are considered stable and changes are smaller and less sensitive. CacheFS being in the kernel might mislead people to rely on a filesystem that is still experimental and under heavy development. Personally I am looking forward to see it mature because I’d love to run it on my file storage home server when it is stable enough.