I don't agree. The Su-27, which is regarded as a genuine threat to 5th generation planes has been in service since the late 1970s. Modern F-16s have little in common with the first ones that saw service. Boeing has been putting money into F-15s and F/A-18s with stealth capability. Fourth generation aircraft are still survivable and they're still being developed on the field.

There's even a school of thought that suggests that when your enemy is a bunch of dudes with death wishes and pick up trucks, lower tech is an advantage. In this kind of combat, the F-35 is at a particular disadvantage. Its cannon may not be able to achieve the accuracy needed for close air support, the helmets' signal clutter is such that an F-35 is actually a danger, and even more fun, not one model of the F-35 can carry more than 220 rounds of ammunition. In comparison, the A-10 carries about 1,100 rounds. In practice, this means that whereas an A-10 could make 10-20 attack passes, an F-35 will top out at 2-4...

Point being, it might not be a very good remaining option and fourth gen aircraft are better in important ways. With some luck, the testing process will solve all of these problems, though some seem incurable.