But there’s a very limited market, and if built with easy/contracted conversion capability into a BCF I think the residual values will be much better, and they might actually sell more
The idea that it is easier to strengthen an aluminum floor beam vs a composite floor beam may have merit, but may not provide as much a cost difference as one may think.
The resale value for a 777 is lower on the priority list than the 20 years of fuel burn from passenger passenger operation.
If Boeing want to simpliy the BCF conversion, they would strategically increase the gage in the composite floor beams to meet the freighter requirement. That would only increase the weight a little relative to all luminum floor beams and would not increase the manufacturing cost much beyond the margin vs the existing composite floor beams and avoid maintaing a second set of Engineering, Fabrication infrastructure (factory space) for the metal beams.
Note that not only Boeing would have to maintain the second line, but their prime as well. It would go against what they are trying to achieve. And it would not be a sale incentive for their dedicated freighter