Twenty years is being extremely optimistic. Someone here once posted that to get to 95% realibility (in technology) is the easy part, it the last 5% that become exponentially more difficult. The accident rate of these UAVs are unacceptably high compared to airliners, and to say that they will be flying passengers around in twenty years is completely unrealistic.
16 computers freezing in 4 months, thats approximately 1 every week. So that is 1 aircraft, being completely out of control once a week, now when you put that into the perspective of airliners, and the amount of aircraft in the air on any given day, and the amount of flights each one does on an average day, then things become really dangerous.
I think it will be a much longer time before we go to UAVs. In the 20's they said we would all be driving flying cars, and robots would replace all work, we astill aren't there.