I think this whole "where will my DS/DD's support network be when we grow old/die" argument is slightly spurious.
Life is full of so many ifs and buts that you simply can't let that dictate to you.
Who is to say that an only child won't end up in a relationship with a person who has loads of relatives and together they'll have lots of friends who will be able to offer them all the practical and emotional support they need?
Even if they don't end up in a relationship at all, they may still have a fantastic network of close friends who can provide them with all the support they need.
The majority of people I meet seem to have just one sibling (or maybe two). If you have just one and relationshs with them end up being strained, or totally severed, where does that leave them?
I have read so many stories on sites like this one, and also heard of and seen in real life, where siblings are quite literally at each others throats over who's doing this for mum and/or dad when they're elderly and who's not. It can lead to so much resentment, ill feeling, nastiness and heartache.
A lot of people have a tremendously rose titned view of what it's like to have siblings. Just because two kids love the bones of one another when they're aged 3 and 5 doesn't mean that' the way it will always stay.
Some people are exceptionally close to their siblings but it's importnant to remember too that siblings grow up and don't always grow together but apart instead. They sometimes move to different areas of the country, or even different countries and often have very little contact. I know of people who feel far closer to their brothers and sisters in law than they do to their own siblings.
I have one sister and have very little to do with her, even though she lives very close by. I can quite honestly say that she, without doubt, has caused me more pain and heartache in my life than anyone else.
At difficult times in my life, my mother my DS and my closest friends are the people who have really been there for me - certainly not my sister.
No doubt the time will come when my mum needs me and things will get tough. My DS and my friends will again be my support network at that time. I can quite easily manage without my sister and, knowing her, it'll be a case of having to anyway.
I feel like an only child, despite having a sister.