Is just right to have only only child?

Nowadays most couples are content with just one kid citing various reasons like cost of education and cost of growing up kids. its right to some extent to give best of everything to one kids. But in this case aren't we depriving our kids of their siblings. We as parents are not going to stay as long as our kids live. Shouldn't they have sibling to play with, to grow-up with. Having just one kid we are killing relations like brother, sister, aunt, uncle and many more. I know it is tough to grow two or more kids, but money and career is not everything. It is the relations which count. What are your opinions about the this.