I think that there is still a place for the B747 and the A380, and most people who say otherwise will typically have either US or Euro blinkers on. Frequency is king for dom routes and short haul international, and whilst the B747 has been used on longer dom routes in the past, neither where really designed as short haul jets.
Once you start getting into the medium and long haul market where time zones start playing havoc high capacity is still a requirement, and unless they manage to both speed planes up considerably and quieten them down to less than a dull roar so as not to annoy the neighbours, there will continue to be a market for the heavies.
I don't actually see either pulling the plug. Whilst Boeing has more to fall back on in the 777 if they did pull the plug on the 747, the 747 is still an icon. Airbus doesn't have a 777 to fall back on, so until they have one that's ready, we'll still see the A380 out and about. FWIW, the A380 is heavily pushed as the flagship product (along with B777's), but I don't think it'll ever achieve the icon status of the B747, that icon status is something that may actually help pull the B747 through.