Like English prepositions are any better :)
"He waited for the bus". for = because of, for the benefit of? Not here!
"I'm going to come on Tuesday." on = on top of? How can you be atop a day?
"Dinner will be ready in ten minutes." in = inside? How can you crawl inside a minute, let alone ten of them at once? And why isn't it "after ten minutes" since it's not ready inside the period of ten minutes but only at the end of that period?
Metaphorical prepositions don't make sense, and are notoriously difficult to translate because different languages often have different metaphors. (For example, in German, you wait "onto" someone rather than "for" them, and you are afraid "before" someone rather than afraid "of" them.)
As others have pointed out, a more normal Esperanto translation for "Let's go upstairs" would be "Ni iru supren." I was given the longer Esperanto version, "Ni iru supren laŭ la ŝtuparo," and had to arrange given English words to translate it. I would have said something like "Let's go up this stairway" but there weren't enough words to insist on the method of reaching the upper level. The real point is: the idiomatic usage of the English word "upstairs" simply means "upper floor," without any insistence on a staircase. The problem here was caused by the English idiom, not any Esperanto difficulty. :-)