I stand to be corrected but I believe the capacity restrictions are due to noise abatement regulations dating back to the 1950's, not the physical proximity of the two runways.
We may be talking about two different things here.
Heathrow has an ATM limit (480,000 per year), set by the Government, which is indeed for environmental (noise & emissions) reasons.
The physical capacity of the runways, on the other hand, is limited by the spacing between them (just over 1400m), which will not permit simultaneous independent IFR approaches, for example.