The RAF 'wisely' binned its lowest-houred Tornado GRs and then upgraded some of the high time aircraft to GR4. It then failed to do the Tornado 2000 upgrade that would have transformed the OCU GR1s into GR4 emulators, keeping hours off the op fleet. And has since managed to exceed the planned hours and FI usage.
The GR4 was planned to last to 2018, and the OSD has been slid back to 2025, which is about as far as it can possibly go.
Would it be wise and cost effective to replace some or all of the GR4s with Typhoons? Of course it would.
Is it going to happen?