I think the high turnover of almost any aircraft type until ~30 years ago was due to the rapid advance of technology and knowledge in materials*, systems, and propulsion - all with regard to both weapons and airframes. Only in the field of aerodynamics has knowledge, albeit sometimes often in theory, advanced little since the 50s; the boffins in white coats had it pretty much sussed by then.
*It could be argued that only the advance in material tech & knowledge has made advances in other areas possible, where advances were only held back by lack of suitable alloys, composites and semi-conductors.
The AV-8B was essentially a completely new design compared to the Harrier/AV-8A; only the general layout stayed the same - although admittedly it will have been in service ~35 years when, (if), F-35B takes over.
Last edited by Willard Whyte; 4th Jul 2012 at 16:58.