The RN didn't move to retire the Invincible class, that was the Government. They make the decisions, the armed forces then have to 'clear up the mess'. The RN had planned to retain Ark Royal and Illustrious long enough to 'hand over' their core crews to the QECs as they commissioned, obviating the need to send RN crews to serve with the USN and gain and retain experience in deck operations that way. Likewise the Harrier pilots had to be embedded with USN and USMC sqns to keep current. The RN plan was a sensible one, the Government screwed it up.
The high attrition rate for the Harrier was mostly early in it's career, a situation improved over time. with experience and training. When first introduced to USMC service as the AV-8A, the corps posted only qualified test pilots to the first sqns. They reported the aircraft as very easy to fly, but then they were the cream of the crop. The USMC misunderstood their reports and began posting ordinary pilots, and in some cases former helicopter pilots, figuring they'd be good with an aircraft that could hover. Indeed they were, but they had been streamed to helicopters in the first place because they weren't good enough for fast jet ops at 500knots, and so many of them flew into mountainsides in training. THAT's where the high attrition rate reputation mainly stems from. In later years it was as good as many platforms still in service. It certainly wasn't as bad as the F-104 Starfighter. As for the age of the airframes. they were sti;; on average ten years younger than the USAFs F-15C Eagles which are STILL flying in the front line and will for many years yet.
Fly by Wire is just a different way of doing things in an aeroplane, nice to have and certainly with advantages, but not the only way of doing things. Russian aircraft like the Mig-29 family and the SU-27 Family do not have fly by wire and seem to manage ok...
