As wonderful as CFRP is said to be, stretching it to the projected 787-11 and 787-12 lengths might have penalized it with significant extra weight - just like the A340-500 and A340-600 stretches did to the A340-300. New wings would have been a given, likely with a span similar to the 777X with folding wingtips. And new undercarriage would have had needed to be developed to support both the higher operating weights and to raise the airframe off the ground to allow the fitting of the GEnx and/or Trent XWB. All of that would have driven up the structural weight, as well.
So it could have ended up being the 787 stretches were also more heavier than the A350-1000 and might have been as heavy or heavier than the 777X.
I can buy maybe as heavy as the A350 but heavier than the 777X - never.
The 777X is 244" in Fuselage height - many think the 787 is a lot smaller - but its actually 234" in height - only a 10" difference (A350 is 240"). Carbon supposedly has an advantage in stretching - in terms of less weight penalty than Aluminum and that given the height difference which is one of the largest determining factors, is not that much, stretching 787 to 80m does not seem that impossible.
The 787-10 is already 68M long. A -11/12 (say maybe 74M and 80M) would arguably have a longer Wingbox - call it 2m - so you are talking about an extra 2M in front and back of the wingbox on the -11 and 5M extra on the -12.
The A350 seems fine with a 65M wing at up to what 321 or higher? Supposedly the existing 787 wing is good for 280T - 65M should be fine for an -11/12 and no heavy/expensive folding tips. XWB class thrust seems like enough.
787 should have a lighter nose/cockpit section. 787 Electric architecture is lighter. Empty weight of 781 is what 135T - 777X is what 180T? Yes the 787-11/12 would have gotten heavier - possibly into the 155-165T ton range, as it would be higher capacity than A351 (at least the -12 would be.). Lighter - less thrust - less need for higher MTOW to meet range targets.
Therefore taking all the above into account - an 75/80M 787-11/12 should have definitely been lighter, sooner to market and less cost to develop. Boeing blew it.