You think this is expensive AZ - try having, maintaining, training, operating, etc.... multiple specialized aircraft - ALL costing an incredible amount of money to develop with all of the bells and whistles and then think about the cost! You want to talk mind staggering!
Similar airframes are a good thing. Current example - a Growler.
XX exchangeable parts, maintenance cycles, parts, training, equipment, etc.... with a majority of the rest of naval aviation. HUGE costs savings in the long run no doubt.
Why do we try to to fit more and more and more into every airframe so that it can do everything and more to the Nth degree? There is a movie out there that demonstrates this exceedingly well. It is about the development of the Bradley Armored Fighting Vehicle. Maybe someone else here will remember the name or do a Google search - but it point out the bureaucracy that gets involved and how a good idea and go down the crapper (and we won't even touch on special interests!!). It was tongue-in-cheek if I remember right - but look beyond and you see the truth in-between the scenes!
As far as testing them in a short amount of time and getting them out the door....yep - and there have been some excellent examples where that worked - my favorite being the A-4 SkyHawk; and there have been MANY more examples where that has failed miserably (think of the many different 50-60s era planes that hardly made it past the testing phase). Crewdawg mentioned a lot of accidents. We are not prepared to go back to those days where military aviation was losing over 500+ airframes a year! Think about that loss rate!
USMCFLYR