Replacing all airplanes with pilotless planes is likely as a bad plan as replacing all car with driverless cars. Its not the a single step, nor has all situations which a driver/pilot has to deal with been automated to the point in which a computer can do all the work successfully.

It is however a great goal, one which spells safer, faster, and consecutive safer technology with every step. Modern cars already has some forms of brake systems which can take control away from the driver during a crash-like situation, so the same concept for planes could be applied if it can be made safe enough. We also have kill-switches in cars which can be triggered remotely, so I could easily see a similar system be implemented which locks a plane into auto-pilot with a per-determined height and GPS direction.

Neither will replace pilots overnight, so they continue having low blood pressure and know that they still got a job tomorrow. Flying in bad weather, imperfect position technology, security, and complex traffic is going to take a long time to solve.