If your main use case is the desktop or embedded, UFS is perfectly fine. If it's enterprise storage; ZFS is second to none obviously.
You cannot remove UFS2, because lower-end systems need that. XFS and ZFS are heavy on CPU and RAM, respectively. Unless they want to cut off all old laptops, and other lower-end use case scenarios, they have to keep UFS2 in the kernel. Otherwise, they need to change the slogan to, "The Power to Serve 8GB+".ZFS is bloated
ZFS is going to remain in the kernel too. Some say HAMMER2 could be a better option. However, it really does not matter, which one is the better one. They will not remove ZFS from the kernel. ZFS is the FreeBSD main selling point -- beside the BSD licence, of course.
Therefore, introducing another FS to the kernel means addition, not replacement, i.e. three filesystems, side-by-side, in the kernel. UFS2, ZFS, and XFS for example. This will not happen either. Either you need many new kernel developers to implement, test and debug it -- for a very long period, or you have to hire developers. I do not bet on the former, and the latter needs money, which according to the latest fundraising it is an impossibility. And even if that happens, then you have larger kernel, more bugs .... To conclude, for better or worse, I am sure that both UFS2 and ZFS will remain the default filesystem in the kernel, for a long time.
P.S. I'm not a ZFS user, I don't like it, and I'm not defending ZFS. However, the FreeBSD Project policy on keeping and promoting ZFS is understandable.