Another factor to consider is the capacity of other Airports to receive diversions. Years ago there was probably plenty of slack in the system, but with the burgeoning expansion of Locos etc, places like Stansted, Luton, Birmingham, Bristol etc, (to name but a few!) are getting much busier with their own traffic. 'Sods law' will eventually dictate that Heathrow's blocked runway will coincide with the alternates' rush hours.