The 747 was designed with the upper deck so that the main deck would be unobstructed when used for cargo. Everyone thought that SSTs would take over within 10 years so the 747 was designed as a cargo plane. This has nothing to do with lower deck cargo.
This is absolutely correct and I didn't claim the opposite.
What I did claim was that Boeing did study a two-level, a la A380, design.
of the reasons they didn't go with it, but instead went with a single level
design, was because the latter gives you less passangers per plane "slice",
if you want, hence more opportunity to add cargo in the belly.
Other reasons included what you mentioned and also coordination of
the evacuation slides.
I had seen a documentary sometime ago with one of the ex-747 designers
talking about all this... and, no, it was not on the History Channel!
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