So my question is, is this type of ATCO workload ( that saves a couple hundred grand a year for Air Services) likely to be a factor in an aircraft accident within the next 30 years?
It has already been a factor. E.g., the Comair 5191 wrong runway takeoff crash at LEX.
Due to his schedule the controller was working alone after sleeping only 2 hours in the past 24. And the crash occurred on the "wrong side" of the controller's circadian rhythm -- he would normally be asleep at the time.
Since he was alone, the controller cleared Comair 5191 for takeoff before the aircraft had reached the intended runway and then attended other workload (administrative tasks) at the tower. That was a missed opportunity to notice that the aircraft had lined up on the wrong runway.