More on "One and Only"

I know two people who have one son who has an autistic spectrum disorder, and he is in each case an only child (well, I know the boys have fathers too but it is the mothers I know - so I suppose that's four people).

In one case they struggled to have children and their son is a surprise, late life child (don't know them well enough to know if fertility treatment was involved). In the other case he was just a surprise, which led to a rapid decision to move in together, and finally get married after he was born. Both are clearly loved, nay, doted on. But I'm not sure if I'd feel differently about having another child if the first had some kind of disability, or an illness that might need extra care.

If it were me, I think I would worry about a child that would need extra care, having no relatives once we reached old age. I'm sure that some selfishness - a child who needs care themselves is not going to be able to care for me - comes into it. But would it be fair to bring a second child up who knew that they would have elderly parents and a sibling in need of care? And, given that most parents have some kinds of expectations, but tend to reduce them for children who are less likely to meet them, would the other child then have all those burdens on them alone?