You are probably right in terms of dev and support overhead. I would imagine that should they decide to add it, the process would be similar to XFS; in that, they would add support for it in the kernel and call it "experimental" like they did with XFS in 5.2.
XFS is a fairly simple filesystem. That said, the issues they had were integrating it in VFS. There are still some bugs at that layer today, albeit minor ones. In 5.x, most of the issues were race conditions in VFS and raid controllers (sas bus resets and such). XFS itself is actually very mature but any time they add a new FS, the VFS layer requires a lot of work.
XFS is a fairly simple filesystem. That said, the issues they had were integrating it in VFS. There are still some bugs at that layer today, albeit minor ones. In 5.x, most of the issues were race conditions in VFS and raid controllers (sas bus resets and such). XFS itself is actually very mature but any time they add a new FS, the VFS layer requires a lot of work.