GR4 does things that the FGR4 can't do (yet, possibly never?). The harrier force, due to its size is probably thankful for the respite from continuous ops of the same type of mission so the guys can get back on their game. I imagine operating from field strips or low level AI trips haven't been flown in a while, yet they are all probably subject matter experts at CAS.
It is a numbers game. Typhoon as a force is still small. If you compare what it has taken on from the F3, there's not much flex to do anything else until the rest of the jets are built and the pilots trained up. It all takes time. Even after then, a multi-roll pilot is likely to be more of a jack of all trades rather than a master of one. GR4 should be around for a loooong while yet.