It was the JAST office that reached the conclusion that a single aircraft could meet all service needs. This was actually an idea that originated with Boeing's internal studies.
Well, how'd that work out for Boeing ... uh, not so good. Fruits of the poisoned tree, perhaps?
Also, JAST didn't occur in a vacuum.
ASTOVL merged with JAST merged with CALF and we eventually got the correct acronym out of the deal: JSF. Alphabet soup. Let's remember that this was going on as the Aspin Dec Def reign began -- oops, that only lasted a year -- and Clinton had replaced Bush, and the increased reductions in defense spending (the slope of the ramp down increased form the original Cheney-Bush model) hit the operating and acquisition systems, and programs, with increasing savagery as the 90's went on. The Roles and Missions wrangling that came after the Desert Storm deal were still alive and well in the mid 90's. I got involved in the air power/air doctrine wars to a modest extent for a few years during that period, and you could argue that the one thing the services could NOT agree on was ... anything.
Heck, as modest a program as JPATS (Now the T-6 Texan II) was a stew of epic foulness.
The JSF (as a program) was a child conceived in a clusterfcuk. No wonder it's got issues. Jast sayin' ...
PS: it's a testimony to the people in it, and their efforts, that it is still alive and growing.