Even entertaining the thought for a moment that the F35 is canceled, there are a couple of problems.
The current generation of manned fighters is limited, and there always needs to be the sad evolutionary factor of having something else being developed to keep and edge.
The unmanned fighter thought is a bit far away in my book, and there is always the have all your eggs in one basket scenario. Imagine developing a whole fleet of unmanned fighters, only for your enemy to have a way of disabling the remote control systems. Imagine if you didn't know about it until the most inconvenient time!