In our case, I think our oldest would have loved to have -more- siblings (more rugrats to boss around, yk?)! Actually, quite seriously, #1 attracts children of all ages, but younger ones swarm. If we spend any significant amount of time in a place with some young children in it, an entourage will collect around #1. I think that affinity would have been well-met with a few additional younger siblings.
There is also value in contemplating the difference between giving your kids the same attention, and attention of equivalent value. We try to give our kids what they -need-, which isn't necessarily the same for each child, because, hey, they're different people. And we've also tried to instill in them a value of kindness and compassion that starts from our family, and extends to the family of humanity. Where kindness does what is in the best interest of the recipient, and not just what the giver perceives as being nice, so that it is entirely individualized, begins from principle and not simply pleasing, and also respects the agency of the one served. We don't want those who need more of something to feel defective, helpless, or entitled, nor those who need less of it to feel neglected or superior.
These compromises, balancing acts, or, as I prefer to view them, complementary relationships, are key to a civil, compassionate society. Sibling relationships, and the parent-child dynamics of a multi-child family, are a terrific sheltered, facilitated workshop in which to learn how to develop them.
Sorry, I seem to have wandered a little off-topic. And, of course, onlies can find relational networks in which to learn these skills as well. Mainly, I mean to say that this view of family has made it--I won't say not a sacrifice to parent multiple children, but--of much higher value to do so than anything we may have given up.
