Again, look at it from the perspective of a user or system administrator rather than a kernel developer. Yes, someone has to remove a bunch of easily searched for and removed code and rerun their test suite but that's a handful of people for a very small amount of time compared to the number of people involved with something which will make a major change on hundreds of millions of running systems. More critically, the development work is completely under Apple's control and shipping can be delayed as long as it takes to complete testing but once they ship a filesystem it will very quickly reach the point where it will be in the wild for years even if they immediately ship a new release which takes a different direction.
The question isn't whether Apple will force this on users soon or without notice but rather the observation that they're going to be very careful not to do this more often than actually necessary. That's why despite having implemented and shipped ZFS in the 10.5 beta series they removed it prior to release. Sure, it might have gone okay but if that changed later there would be no easy way to go back without forcing users to migrate existing data and if that hit a licensing/patent case, the other side's lawyers enjoy the extra leverage which that would give them.
As an aside, “There are many filesystems in the kernel” is technically true but misleading: HFS+/HFSX is used constantly on every device, FAT/ExFAT are used regularly by many users, and everything else is a rounding error. A few Mac users use NFS or SMB, even fewer use UFS, etc. and no iOS device uses any of those.