The U2 is a very versatile manned aircraft...technological advances help to reduce size, weight and power impacts on the aircraft allowing it to continue to expand its mission capability.
Still has about 2/3s of its airframe life available.
Drones have their place, but having a pilot able to switch between mission systems or a planned mission route in a few seconds vs. a complete re-progamming that could take hours has its place as well.
Dragon Lady...she's still rockin'.