The F-35 is so expensive too few can be built and none will be risked in combat, and if they were lost in combat, they could not be replaced.

I have no idea what the next major conflict might look like, and nobody else does, either. (WW1 and WW2 both showed large swings in fortunes with even minor technological improvements.) But it's hard to see how the F-35 could play a role in it.