I'm going to go out on a limb and say both the 747 and A380 will continue on for a very long time. The 747 will get some further tweeks, the A380 will get a longer fuselage. Airport planners in Asia are already talking about designs for airports and terminals which will see 500+ million pax per year. The new twins are magnificent air frames, but they are not the 747 or A380. Their demise is greatly exaggerated to paraphrase the well know quote.