Not sure anybody is suggesting adding aircraft or pilots only replacing existing aircraft with younger ones that have more modern kit.
John:
More modern kit.
I appreciate that fatigue life on "younger aircraft" is an attractive feature. No question.
The remainder of the kit, be it "more modern" or not, if it isn't in current configuration represents a cost (of non trivial scope) to ingest into the system.
We shall see.
I'd need to do a side by side, system by system analysis to see what "kit" (considering the usual Airframes and Avionics changes/upgrades that the Harrier fleet undergoes during its service life) represents an upgrade, or simply a change.