etudiant
For express stuff, yes, but a heavy long-haul aircraft freighter will have to be able to carry all sorts of loads, not just thousands of parcels and envelopes.
Big difference is also that B777 can get fly to pretty much any airport, and B747 to most of them. Good luck trying to operate a potential A380F for couple of charters into a small-ish airport that isn't certified for it.
A380 is a great airplane that came at a wrong time, and since it makes for a poor freighter, the end is very near. Time to face the reality.