I think the thing people are missing about caring for elderly parents isn't so much that it usually falls to one person anyway, it's the way it shapes expectations.
I know a number of families where it's already clear which sibling is likely to take the lead when their parents get older. However, the only children I know, including myself, seem to factor it in differently. The 'carer siblings' haven't made any significant changes to their lives in advance. I have one good friend who, despite a career path that would have prospered if he had gone to work in mainland Europe- which he would have loved- felt he needed to face up to the fact that longer-term he needed to be closer to home so never moved abroad. This is in his mid-twenties, many years before his parents would need support.
I lived overseas but always thought about career planning with regards to a need to move back in my mid-thirties, which I have done. I know two other only child friends who have had similar worries/concerns since their early twenties- none of us discuss this with our parents, but we know the reality and it is in the back of our minds.
One difference might be we're all only children of older parents. But it is a reason I'm planning on having two. Also- DH isn't especially close to his siblings. But, they are people he has a shared history with. It's so strange thinking when my parents aren't there anymore there will be nobody who really remembers my childhood in that way.
I didn't enjoy being an only child, but I don't think it's a tragedy. If an only child will give you the space to love and nurture your child in a way you couldn't with two, then have one. If you have left I too late fertility-wise to have a second, that's not a reason not to have an only child or to feel bad about it. But if people feel like they should stick with one as a choice because of, say, being able to afford better holidays or private education, I don't think that's a sensible choice: I was super jealous of my friends who shared rooms and went on camping holidays.