I bet the 787 fuselage is too heavy and what you are proposing seems akin to wat the 777x is. Financially makes a lot sense but maybe not what the market wants. The reason for the pax focused 797 lighter oval fuselage rumors.
Financially making sense is the main driver.
Boeing won't do a vanity project like the A380.
I'll just throw up some (hopefully) realistic numbers.
787-3 - 300 passenger - 9ab - 4500nm
787-2 - 260 passenger - 9ab - 5500nm
797-9 - 270 passenger - 8ab - 4500nm
797-8 - 230 passenger - 8ab - 5500nm
Let's assume two 787 versions cost $5 billion total.
Let's assume two 797 clean sheets cost $15 billion.
Let's assume the 797 versions are cheaper to produce due to new manufacturing technology and have 5% better fuel burn per passenger.
Now you must estimate how many more 797's will be sold due to the increased performance and lower price.
You also must estimate how many more 797's need to be sold to pay for the extra $10 billion in development cost.
We could estimate 500 lightweight 787-2/3's sold.
We could estimate 1000 cleansheet 797-8/9's sold.
That means the new 787 family members needs $10 million per frame to pay for its development. Where as the 797 family needs $15 million to pay for its development. This reduces the price gap between the two.
Going even further the new 787 versions would reduce the price of all shared systems on the 787-9/10. This is another bonus and increases the profit margin.
You can skew the numbers to make either option the winner in the long run. I have no doubt Boeing has looked at both options and has crunched the numbers.