What Difference Would it Make if You Had Not Existed? Here's a fun thought you're inspiring in me:
Supposing that most of us are not irreplaceable there is a sense that even if the me that I feel myself to be right now does not exist that I could, for all intents and purposes, exist -- my individual ipseity would be bound to another that I do not experience, but the place I hold in the world would still be fulfilled.
***
But then when I think a little more locally I think that we aren't exactly replaceable one for another: it's the particular relationship between myself and my loved ones and such that's important. What is not important there is one's effects on The World Scene, as if the world were some kind of testing grounds to demonstrate and pursue our own perfection.
Rather it's smaller, gentler, quieter than the world events at large: But also meaningful to me.
The first answer that come to mind
@Jack Cummins was "Supposing I could somehow see this world that doesn't have me in it I doubt I'd care because here I am in my world being myself. My existence matters to me, but that's only after having been brought about, whatever that consists in."
In a way my existence mattering to me, at the other extreme, is a social act: There's a certain point where you are expected to brush your own teeth, for instance. That amount of individual care is expected by our loved ones: they want the best for us as vice versa in the ideal sense at least.
But insofar that I cease to engage in the world with an eye towards others then we run into the story of self-improvement, comparison towards others, achievement: In a way a new kind of sociality, but one which is a dance of individuals pitted in competition with observable metrics such that we can eliminate the useless amongst us. (And crow when our enemies are defeated)