Don't worry Boeing747-400, I'll help you out!
Whatever 747 derivative Boeing puts on the market, it will only be a relatively short-term solution in my eyes. The A3XX product family has a lifespan of about 50-60 years (like the 747), and it has a lot of growth capacity.
It's as simply as this: If Boeing wants to offer a 600 passenger plane (3 classes), they would have to develop a true NLA doubledecker like the A3XX. This is because the 747 cannot be stretched longer than approx. 85 meters since 80x80 meters is the standard for which all large airports are being prepared (parking and taxi-wise for example).
All proposed A3XX models fit within the 80x80 m box and offer a capacity of up to 650 pax in three classes (A3XX-200). A 747 derivative won't do that.
As long as the market requires less than 550 seats, any 747 based aircraft is a big competitor to the A3XX. And I don't think the "old" design could be a real disadvantage for efficiency, only for passenger comfort (more space on the A3XX).
If Boeing has to come up with its own doubledecker in about 10 years, which would cost 10-15$ billion to develop, is it wise that the invest around 4$ billion now in a 747 stretch??? I doubt that.
Maybe Boeing's strategy to wait what Airbus had to offer was wrong. They probably lost valuable years of development work so that they are now not able to offer a plane like the A3XX.
But only the future will tell...