Any airline can fill any aircraft on any route.
It's just a matter of what price they can get for each seat.
One airline makes up the bulk of the A380 order book.
This airline has found that it can hub enough traffic through a single well placed airport to make this aircraft work well.
Most other airlines have ordered a relative handful of A380s.
The total order book for the 747-8i is embarassingly small.
The future looks to be in big twins. For slot constricted airports, higher fares will be used to deal with demand. There are always nearby alternatives.
If you really must arrive at LHR, you'll pay for the priviledge.