The STOVL capability of the F-35B comes with a cost. According to the Congressional Research Service, the projected flyway cost for the various models are :
F-35A : 77.7 million USD
F-35B : 105.5 million USD
F-35C : 89.7 million USD
F-35A : 77.7 million USD
F-35B : 105.5 million USD
F-35C : 89.7 million USD
As for 'generations behind', this is the whole point and the linchpin of global military stability.
https://en.wikipedia.org/wiki/Graham_T._Allison#Thucydides_T...
And oddly enough, it's the model that drove most of the program's requirements.
Why civilians in congress thought it was a good idea to have a one-size-fits-all Fighter/Attacker is beyond me, but then to effectively allow Marine requirements to pollute specifications for the two branches of the military that operate the most aircraft is another thing entirely.
I feel that explaining why the civilian government thought it was a good idea is one of the easier things to explain in the entire F-35 saga.
Another question is why was the F-4 so successful across all branches of the US military?
A key difference in the F-4's gestation is that the Navy pretty much single-handedly drove all of its requirements. (The USAF didn't really want any part of it, and in the end it was shoved down their throats.) And if you think about it, the Navy's requirements tend to be more stringent. The Navy requires more range (blue water ops and less tanker support), more durable stealth coatings and features (shipboard life is hard on an airplane), better fatigue life (carrier landings take their toll), more corrosion resistance (saltwater exposure), simpler maintenance (all work done in crowded decks with limited ability to fly in new parts), lower approach speeds (the slower the plane lands, the more flexibility the captain has in how s/he drives the boat) and so on.
So when the F-4 was designed, it was born out of a relatively (!) unconflicted set of requirements, rather than being a grand compromise on behalf of every customer.
This is how I think the F-35 should have gone -- the Navy should have set the requirements, the USAF should have been allowed to add their own refueling gear, and if the Marines wanted VTOL that bad they should have gotten together with the Brits and modernized the Yak-141 airframe to which Lockheed bought rights, as any other alternative would have been too expensive.