There are essential comms problems that will face any unmanned system that tries to take over the F-35's role. In the absence of a solution that is extremely low latency, immune to interference, and undetectable as radiated emission, you'd have to lean on computer AI to run the bulk of the mission autonomously. Nobody is ready to throw the switch on that.

A possible solution is to augment flights of manned fighters with unmanned drones as missile/bomb trucks. This gets you the force multiplier while keeping all communication short range and within line-of-sight. But you still need a stealthy, survivable manned fighter.

Overall it's a bit like saying why bother with a new generation of conventional cars when universal self-driving is right around the corner. Maybe it is, or maybe it isn't quite as close as you think. These are questions that are more appropriately asked of the N+1 generation fighter that may or may not be on the drawing boards at this time.

P.S. I've noticed the F-35 takes flak from both the extreme technological pessimists (drones/fighters can't take over the A10's role, low & slow manned flight is the only way to do CAS) and the extreme optimists (drones will make all manned flight obsolete next year). Seems to me neither faction has a very strong grasp of the state of military tech.

How would you fly a drone in a heavily jammed area?

Who says you can't replace the cockpit with a computer?