recce_FAC; the GR9 is going because despite comments about it being better than the GR4, the GR4 can do the job to the required standard; and do the recce side of the house better.
We can't afford to have 2 jets that can do the same thing. If the GR4 couldn't provide anywhere near the capability the GR9 could, then I'm sure there'd be more kicking and screaming. As you've said though, it's doing a good job.
If you have any specific complaints about how the GR4 works in the CAS role, then do something about it; everyone flying it right now would be very receptive to some constructive criticism, especially if it means that people calling for CAS get a better service!