Neither. It is the way it is.
Single children get undivided attention from parents, which is important in today's fast-paced life. Nowadays there are greater demands not only on parent's time, but also on the child's time, in the form of school activities, extra classes, drawing/dance/karate classes. Two or more kids lessen the parental attention.

I strongly disagree about the companionship part of the multiple children argument. Nowadays, when children of two years handle mobile phones and global connectivity is a part of everyone's life, there is no question of "being lonely".
Moreover, how many siblings are really connected nowadays after having families of their own?