Well there's no doubt on the side of the aerospace companies. Anyone can confidently offer to engineer you anything given sufficient time and resources, and a 'plane' that is actually a family of somewhat-related airframes in different configurations seems like a plausible way to be all things to all people given virtually unlimited budget. A certain amount of cynicism is required to bid in such circumstances, but money does tend to distort things.
The commissioning side is much less clear. Who on earth would think one size fits all is the best approach? One possible guess is that it's an attempt to CONTROL complexity and cost, compared to having a succession of over-budget programmes and small orders for aircraft that cost fortunes to develop. A big bet on technology sharing, and an acknowledgment that the development of each new line of military aircraft has already become staggeringly expensive.
The F35 program has been pretty much a disaster. But, that said, the concept of departing from a model in which every service branch and every allied country to which we sell military hardware has largely unique equipment for every mission isn't inherently a bad one. But we've apparently discovered that, among other problems with the program, it's harder to come up with a viable common design than many apparently thought.
The bulk of the requirements for the F-35 are perfectly reasonable; a stealthier replacement for the F-16 and F/A-18, which essentially perform the same role but the Air Force chose one and the Navy chose the other. If you could get a next generation light fighter-bomber to replace both, and make it stealthy enough to replace the F-117 at the same time, that would be the perfectly reasonable aircraft. As it stands, the Navy already hedged their bets with the F/A-18E/F Super Hornet, which has been a complete success.
The only thing that did in the Joint Strike Fighter was the requirement to develop a STOVL close-air-support platform for the Marine Corps, to replace the Harrier. That should have been a separate project. STOVL and conventional aircraft have entirely different aerodynamic requirements, and there's no real requirement for a stealth close-air-support platform because it's not like the enemy isn't going to notice the slowly hovering jet plane that's strafing them and shooting missiles at them. But it's so hard to get Congress to fund any replacement aircraft at all that everyone decided to go ahead and put all their hopes and dreams and lobbying efforts behind a single plane.
> it's not like the enemy isn't going to notice the slowly hovering jet plane that's strafing them and shooting missiles at them
I thought the STOVL was designed to enable take off and landing without airfields and aircraft carriers. Are you sure the intent is that it will hover over the battlefield while attacking? Current planes seem to hit targets fine while flying at a much safer hundreds of miles/hour.
It's not going to hover, that would be insane (in fuel use if nothing else) but the USMC wants/needs a close air support platform. A jet that will fly sufficiently low and slow to attack ground targets effectively. Even when the friendly troops are only a few hundred meters away.
That's intrinsically different from the Air Force and Navy whose planes will spend their time closer to 30k feet then 3k feet.
Flight at low altitudes and speed requires fundamentally different characteristics then flight at high altitude and comparatively high speed.
> It's not going to hover, that would be insane (in fuel use if nothing else)
Really nerdy aside: it would also be insane with regards to hot gas ingestion. There would be a very high likelihood that the engine would take in the missile exhaust if launches happened in the semi-jet-borne or jet-borne regions.
The main platform for Marine STOVLs is pocket carriers; littoral assault carriers that lack cats and traps. Quite often they also carry amphibious landing craft as well.[0]
Reducing the number of commonly used planes from dozens to a handful is a great idea. Reducing it to one is not.
But there already are fighters that are widely used. The F16 is used by practically everybody, for example. It's a perfect backbone for a small air force, and still a very versatile fighter in a large air force. But some missions require something more specialized, and it makes sense for a large air force to have more specialized jets for those roles.
My guess is that they don't, but they get to play with cool tech. And hey, the money is flowing so why not try?