Suppose all who are born are assured of an utterly blissful, unending, afterlife. That’s quite a supposition, but just suppose. Add to this the reasonable assumption that one first comes to be at birth, or rather at conception: one does not pre-exist one’s conception either as an actual Platonic soul awaiting embodiment, or as a merely possible individual awaiting actualization. Two assumptions, then. The second assumption amounts to the claim that before one is conceived one is nothing at all.
Given the truth of both assumptions, had I never been born, I could not have missed out on any good things, in this life or the next, and this for the reason that one cannot be the recipient of any good if one does not exist. Call that the underlying principle.
To state the underlying principle in general form: Nothing can give, receive, have, lack, enjoy, or suffer any thing, action, property, or state unless it exists.
And so, although I am alive, and the good in my life preponderates and will (let us assume) continue to preponderate over the bad, and I am assured of eternal bliss from the moment of death on, I would not and could not have missed out on anything had I never been born (or rather conceived).
Is the underlying principle more reasonably accepted or more reasonably rejected?
Assume you agree that it is the former. Now consider someone whose life in this world is on balance very bad, but its badness will be more than compensated for by an eternity of heavenly bliss. Even so, it seems to me that it would have been better had this poor schmuck never been born, and this for the reason that, first, had he never been born, he would never have suffered the terrible things he suffered in this life, and second, had he never been born, he would not and could not miss out on anything good including transcendent goods that would have more than compensated him for his earthly suffering.