If one were keeping the Jag Force (or a small part thereof), the sensible solution would be to take the 2,000 hr ground instructional GR1 jets from Cosford, give them a major, and convert them to 3As (which would cost less than the £450k per jet paid for the current 5-7,000 hr 3As, since many LRUs could be robbed). That this is possible has already been demonstrated (two ex-GI aircraft have been converted straight from 1 to 3A).
And even if you didn't do that solving the Jags leaks and wiring would cost far less, per airframe, than the new rear fuselages required to keep the GR7s and 9s in service.
Nor is it an 'either/or' - keeping an enlarged Jag unit would give the Harrier GR7/9 and GR4 fleets a sporting chance of making it to their planned OSDs, which would otherwise look difficult to achieve without very major expenditure, and would give the RAF a FJ that was cheaper and easier to rapidly deploy, with a smaller logs footprint and that can do recce, TIALD, PWIII, etc. at least as well as either of its complementary platforms.