I think both companies realized that their NB offerings can't be stretched further. And their WB offering can't be shrunk any further and be efficient in its role.
The only thing I will say is this: Single Aisle 787.
If the 787 is the replacement for the 767, and the 757 was basically a 767 with a narrower fuselage, why can't Boeing just do the same and create a 797 that is a variant of the 787, same wings, cockpit etc.