But really, it was just about four-engine planes becoming too expensive to run. Two-engine planes won. 777 burns 30% less fuel per passenger and has almost the same cabin width. And top level became a flop because it's too narrow for a first class cabin by today's standards and all other uses for them make no sense. Top floor existed at all because it was Boeing's entry for a heavy cargo plane competition in which C-5 Galaxy won: it was meant to be a cargo plane with a small - top floor - passenger cabin.
I think the top floor is there because the crew cabin has to be high so the nose can swing up. The cables and wiring from the cabin can't be easily disconnected to allow such access. You will notice other large cargo variants of airliners load cargo only through the side of the fuselage.
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