maxalt
With 5 minutes to go on your colleague's slot I would say it would be debatable if you could have got airborne within the slot window anyway. It takes a good ten minutes to push, start and taxi to the holding point at the regional airport I work at, let alone Heathrow.
The reason that a lot of slots go to waste is because operators are not realistic about meeting them. As soon as it becomes obvious that a slot is not going to be met, a Delay message should be sent. This serves two purposes, first, the delayed aircraft gets a more relistic slot, second, the original slot is freed up for other flights who are waiting on the reply from a Ready message. Had this been done then there is every chance you may have been allocated your colleague's slot.
At my place we have in the past managed to transfer a slot after a quick call to flow, in one case the crew on one airline offered to swap with another airline. In the end though I think it really depends on fairnessand how many people have a Ready message in the system. After all, how would you feel if you had been waiting an hour on a Ready message to find that someone had jumped the queue ahead of you?