Well obviously I'm not going to popularize a method of immortality that requires killing people! That would defeat the entire point!

Actually...

We know from canon that a Horcrux is really just a backup copy, so that on average you probably only about double your lifespan by making one, at best. But it seems to me that based on what Harry has been told, he should believe that a Horcrux makes you immortal and unkillable. Given this belief, in the absence of any better way of achieving immortality, the spell is considerably better than baseline.

Set up a Horcrux clinic. A pair of people come in, fill out some paperwork, flip a coin in the presence of a notary, and the winner of the coin toss kills the loser to make a Horcrux. If nobody ever cast the spell, the loser was going to die eventually anyway, and if you do it when both parties are close to dying of old age (though this would probably be a needlessly reckless strategy, all things considered) then the loser doesn't even lose all that much.

You could save half of the wizarding world from certain death.

Oh, and Harry didn't even ask whether it might be possible to substitute an animal for the human sacrifice, or make a portrait of the caster and use that as a substitute, or Transfigure a rock into a copy of the caster (under heavy precautions, of course). He displays an irrational degree of revulsion at the idea of killing people. One gets the idea that he would let five die in the trolley problem.