Drones will work in a low threat environment, where 'work' is defined as doing a range of things that we certainly find useful. There are a number of things that manned aircraft do that nobody, as far as I am aware, has managed to outfit and program a drone to do. I suspect that there are jobs that need doing that drones will never be able to do.
This isn't be all that odd, no weapons system yet has been able to do everything - drones are no different. Whether a drone will survive in a high threat environment is more open to argument, I can think of several ways of disabling or destroying a UAV, ranging from the hard kill of employing somebody in a P-51 to go shoot them from the sky to the softer but also permanent kill of attacking the command link, or tossing a grenade into the control van. (Okay, it'll work the first time, after that I'll need a hilltop and a mortar).
I find it sad, more than anything, that people seem determined to see the evolution of combat as something that involves discrete steps rather than a continuum of change... I bet the countries that blend UAV use in with regular manned aircraft will do rather better than those who go UAV and sod the pilot.
Dave