Less than 1% of LHR movements are A380s.
i got that point when you first made it, not that it wasn't something I knew already.
Heathrow is already slot restricted with no room for expansion without a third runway - something you may have missed. 9/11 and the 2008 recession co-incided with a migration fron 4 engines to twins on the grounds of economy and the slow decline of the 747 in passenger service. Airlines which flew the 747 and migrated to twins are flying aircraft with fewer seats. The bean counters like twins, even with fewer seats, on the basis of reduced cost per seat mile.
However traffic is now growing and with smaller seating capacities offered the demand will be for more flights into slot constrained airports. Common sense says the equation doesn't work, something has to give.