I guess it depends on whether the "universes" are really separate as you say. If we could detect the existence of other universes, it would mean that there's so

I guess it depends on whether the "universes" are really separate as you say. If we could detect the existence of other universes, it would mean that there's some sort of relationship among universes. If we cannot detect them at all, either they don't exist or they're completely separate, and our inability to detect them won't tell us anything about which of the two scenarios is true. When we're talking about "parallel universes", of course the word "universe" no longer means the sum of everything that exists. It has to be redefined as a suitable subset of existence, perhaps similar to the way we've learned to distinguish the "observable universe" with the universe proper. How we ought to redefine it will depend on what we find out (or not) about parallel universes (or lack thereof) in the coming decades and centuries.