Now, now, the Mustang II was just fine at what turned out to be its ultimate purpose.
Being a donor car and providing steering/brakes/front ends for kit cars.
He ignored the Fox body as well, which was on par with the Chryslers he would build in the contemporary time frame. Mostly he stopped making cars after the Ford deal ended in 1970 because he was busy swimming in the GIANT PILES OF CASH he'd gotten from Ford for beating Ferrari and from selling the Shelby Mustangs.
It was only a personal appeal from Lee Iaccoca that got him out of retirement and into working for Chrysler.