Now that it's clear that the Harrier is indeed retired and there is no other fighter, I do not understand why would you want to retire a plane (even if it's old) if there is nothing to replace it? It's not like fighters are not needed.
retired because the airframes were all old and shot. the choices were (in ascending order of capability/cost):
order a new batch of basically obsolete aircraft just for the 6-8yrs until the F35 turned up,
develop a less obsolete version of the harrier and be stuck with it for another 20yrs, or
accept there will be a 6-8 years gap in capability and wait for the F35 to replace it.
given that a fleet defence scenario which couldnt be handled by missile destroyers is very unlikely, and that we already spend more than anyone except US and China, so dont really have any more cash to throw at mil budget, #3 was the best option.