Large segments of the erstwhile British empire, from the North Americas to Australasia and all points therein, still need to head into London to sort out a variety of their issues. This one factor alone continues to make Heathrow/London something like a "compulsory waypoint" on really odd routes like:-
India - Pakistan - Bangladesh
USA - Ireland
Cyprus - Cyprus
South Africa - Zimbabwe
Chagos - Mauritius
Papua NG - Australia/New Zealand
Timor vs Timor
Singapore vs Malaysia (and till a few years ago, Singapore vs Singapore)
Sri Lanka vs North Sri Lanka
Saudia Arabia by itself
UAE by themselves
Egypt - Israel - Syria
Kurdistan - Turkey
Large parts of Africa
Eyl to Luxembourg
And so on and so forth.
In addition, the crowds at T3/T4 and T5 are really thin nowadays, even the evening peak hour security rush has more people at Krispy Kreme than at the Security Line at T3, for example, so maybe a 3rd runway won't really ever be required.