I think that it's relative. Eddie says that if there are any other Stephen Kings, he senses that they're not important. And I think that's true -- the multiverse will end if they don't save that one man at that one time, even if they could go back in time and save a different version of him -- but I think the truth of that depends on the circumstances: he's the only important one from their current point of view. It's subjective to the larger story. No other version of the ka-tet could effectively rescue this SK. My theory is that many different worlds link to one another and what is most real varies according to where one happens to be standing.