The cargo variant was their hedge bet, I thought that was clear. I know they went all in but had the passenger version failed they had a fall back.
Don’t think I’ve ever heard it put that way. Found this on Wiki:
“In 1965, Joe Sutter was transferred from Boeing’s 737 development team to manage the design studies for the new airliner, already assigned the model number 747.[9] Sutter began a design study with Pan Am and other airlines to better understand their requirements. At the time, many thought that long-range subsonic airliners would eventually be superseded by supersonic transport aircraft.[10] Boeing responded by designing the 747 so it could be adapted easily to carry freight and remain in production even if sales of the passenger version declined.”
https://en.wikipedia.org/wiki/Boeing_747
1965 was pretty early in the program so it could be that they considered that too be a hedge. Air cargo was a small business in 65 so if it was a hedge somebody had great vision.