The rational of having just the one type of plane makes sense if you're Ryanair (who only fly Boeing 737-800). Of cause they only have ONE role for all their plane. Having just one type plane take the role of multiple others just gives you all of the complications of each of these plane all rolled up in one, plus a series of its own unique issues.
I understand that Lockheed Martin just tries to deliver a product that request, but that's really the issue with many government contracts. You get what you request, and if someone tells you "Yeah, that might be a bad idea", the contract is just rewarded to a less honest company. It would be a bit shocking if no one with Lockheed Martin questioned if the F-35 was a good idea.
At some point someone is going to dictate that the F-35 is done, true or not. The US will fly them for a short while and the other nations, that can't afford new planes for another 20-30 years will be stuck with this thing.