"but by it's very nature a life without children is less, in the same way that a life without a parent is definitively less"
See, this is bad logic. Not having kids is in no way comparable to not having parents, if for no other reason - and there are many other reasons - but for the fact that the experience of being a child catered to by parents is by no stretch of imagination comparable to that of being a parent catering to a child (and I will not belabor this point, it cannot not be self-evident). But even if those two experiences were, for the sake of this argument, comparable (they're absolutely not) - what of the people whose parents died early in their lives, or left them for adoption, and who also don't have/can't have/don't want kids - are those lives automatically capped at how happy they can be compared to those of the people with children? To me, arguing that is very naive and reflects a very simplistic, unimaginative view of what life can be. Every mind is, to a lesser or greater extent, a black box, so making blanket assumptions or claims about what someone can or cannot experience, especially based on arbitrary factors like familial relationships, is guesswork at best - never a statement of observable or measurable facts. Not having kids is not a disability or an illness for us to assume that it invariably has an adverse effect on happiness or on the capacity for happiness - or on the maximum achievable happiness.