[Paleo Conservative]

What most people don't know is ?
On a new airplane design and program, the first few planes are only for test purposes.

The 747 prototype -- which actually was a production aircraft -- was the only 747 not delivered to a customer. All the other 747's used in the test program were eventually refurbished and delivered to the airlines. The 777 prototype was refurbish and sold to Cathay Pacific Airways. Considering the first 777 test flights started 11 years ago, why can't Airbus build a prototype that can be deliverable as an airliner?

[investigateworld]

If you can help out a non-engineer here, can the 747 frame be tweaked out even more, or will the production line close?

[Prophet in the wilderness]

Well, the first prototype of the 747 was only used for static tests, and the 3 rd 747 was literally broken into pieces while they tested it for it's engineering limits and fatigue tests.
The first 747 was also used in other tests, as in the flying testbed platform for testing the first engine for the 777.
The first 747 prototype is loaned out to a museum now ( I wished the Smithsonian - Air&Space Museum would have gotten it).

[Prophet in the wilderness]

You might be right about the 777 , but, at least one of the new program or new airplane design is sacrificed ( take one for the team ) is tested for fatigue and broken into pieces.
I do know, at least 2 of the 747's never went into regular service.
I do have a great book on the history of the 747 ... it's called ( The Boeing 747 - design and development since 1969 ( really, since 1966 ) by Guy Norris and Mark Wagner.