Your post indicates know more specifics than I, but I think what we have to remember is that sometimes the replacement of an aircraft type is made by a competitor, and a smart manufacturer accepts that.
Boeing may not want to concentrate on replacing the 777 with a successor, because it knows Airbus will do that with the 350. Besides, why compete against yourself?
It makes more sense to me to replace an A/C class of the opposing team, or create a new class of A/C. The way Airbus did so well with the 330. The way Boeing did so well with the 777.
When Airbus started to challenge the 747 market with first the 340 and then the 380, Boeing pretty much let it happen, and instead created the 777, a whole new class of A/C that has no direct competitor, and now ditto with the 787. These are products with much bigger market potential.
Boeing has purposely not come up with a clean sheet replacement for the 747 because it sees market potential in other areas, and realises that that VLA market is "niche," compared to the long range twin market, for example.
It's not Boeing's habit to defend it's markets, in my view, but to create new ones. The 707, 727, 737, 747, 777, 787 all did that.
Further, it's been my view that Airbus seems to have focused on Boeing products when green-lighting new projects, which resulted in what I view to be a few mis-steps (the 340, certainly, and possibly the 380 (TBD). I believe that manufacturers should focus primarily on future market trends, not on defending A/C that were designed 20 years earlier and represent the markets and technology of almost a generation before.
Airbus made a very smart choice by making the 350 decidedly larger and with more range than the 787, so that the two do NOT compete directly.
I suspect that Boeing created the 757 and 767 primarily based on what the market was asking for and what technology could produce, and not so much to protect it's 707 market share. The only time a manufacturer should concentrate on the competition is when there is a possibility of creating competing A/C that are too similar, and so the possibility exists that they cannot both be profitable.
That was Lockheed and MD
's downfall with the DC-10 and L-1011.