Against a low tech opponent like ISIS and the Taliban, the F-35 is an overly expensive money pit. In these types of environments, the cheapest, most effective way to deliver a bomb is by the truckload; bombers. Bombers like the B-52 and B1B have vastly longer loiter times, significantly higher payload, and when they're done leveling the side of a mountain, will simply fly to the next job and do it again. Yes, they are expensive to operate, but given how the battlefield operates where there are multiple bombing missions being called at the same time, a bomber is most efficient and cheapest at the job.
Against a higher tech opponent like Russia or potentially China (hope not!), the F-35 will be necessary. Conventional bombers like the B-52 and B1B and ground attack platforms like the A-10 are much more vulnerable against more advanced ground to air munitions. Is the F-35 as efficient as the others at doing air to ground operations? Absolutely not. But they can help deliver bombs where the others can't.
I'm very critical of the F-35 program too, but we have to remember that not all of our enemies are in the Middle East.