I have seen it suggested that what people think of when they hear "a life barely worth living" is a live on the edge of suicide, which is well past not worth living. As such, it's not surprising that you wouldn't want a world full of people like that.
If the lives are worth living, then it seems to me intuitively obvious that sufficiently many can be arbitrarily valuable.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.