Consider also the variation of the problem where you're a surgeon, you have 5 patients, and one healthy person whose organs/blood/... could save all 5 if only you murdered the one

I have the book where that idea originated and the author is very proud of the additional ethical complexities he exposed with this example. But for some reason he never considered the possibility of kill (or allowing the natural death of) one of the 5 terminally ill patients and sharing out their otherwise undamaged organs to the other 4.

I'd say for the sake of argument we're assuming that's not possible. Perhaps the blood types are incompatible in any direction but killing the healthy person [1]... the point isn't why these are the only options but assuming these are the only options what would be more ethical. Same way the point of the trolley problem isn't to say "derail the trolley" but to probe at which of two options is ethical in the nice simple world where those are the only two options - because if you can't say what's ethical in a simple world you certainly can't in a complicated one.

[1] do you need blood type compatibility for organ transplants? I really don't know... But I'm certain there are some compatibility constraints to ground the example in the real world...