That was based on the experience of many Bomber Command crews during the War, and wasn't quite the way you think; very, very often, the pilot had been able to hold the aircraft steady enough for the crew to escape, but as there was no-one to hold it steady for him when his turn came, he died as the aircraft span out of control, preventing him from making his escape.
The cynics would say that the system used in the Vulcan showed who was the most valuable crew member, but that simply isn't true - it's to give the pilot a reasonable chance to get out, and therefore actually encourages him to risk staying at the controls while the others jump.
And if you read your World War Two stories about how crews behaved when they had to evacuate their aircraft, you know that the probability of the pilot cracking out and leaving the others to die is very, very small.
Of course, the Vulcan was originally designed to operate at high altitude, not down on the grass the way it tended to later in its career.