This was on my mind towards the end of last year when I seriously considered the idea of stopping at one child (though it would never have happened anyway, as DH is very keen for another) ...
But then I went through a lot earlier this year with my parents (they're long divorced; both were going through entirely different things) ... during that time the support of my siblings was so important - even my husband couldn't understand or empathise entirely ... so I worried about the burden it could put on Clara on when DH and I are much older, if one of us was to develop alzheimers or a serious health problem etc...
However, having said that, my mother hasn't spoken to her brother (her only sibling) for something like 15 years so having a sibling is no guarantee that they'll always have someone to turn to!
And I can only speak highly of the only children I've known... they've tended to be outgoing, well rounded, achieving types. As far as the spoilt stereotype goes... I can think of a lot more spoilt youngest children (in particular, a few youngest girls with two older brothers

)