DH and I have 4 siblings between us and of the 2 with kids, none of them live in the same country as us (one in Scotland and another across the world). Lots of siblings for the parents does not equal lots of cousins close by.
I hate threads like this as it makes me question having an only, despite it being best for my MH - plus DC was prem and pregnancy was one of the worst experiences of my life. I'm also now early 40s (child is nearly 4). Mum guilt is made worse by having moved during lockdowns and having no cousins nearby (although my sibling is hoping for their own baby asap and they live quite close).
I always get in a complete tizz reading these threads as I tend to focus on the negative 'only child' stories, but what I do take from them is that you absolutely cannot call it. Everybody is different, everybody's situation is different, everybody's perspective is different. I grew up in a large family and hated the chaos, I felt that my eating disorder went undetected because there were lots of other siblings with their own problems which divided our parents attention. I was in my sister's shadow for many years, we also hated each other and fought terribly for the most part, she also introduced the concept of eating disorders to me and gave me my first recreational drug (these became my two biggest regrets in life). We get on now but live opposite sides of the world and despite that, still manage to fight when we get together!
Regarding old age, well I'm hoping that we will have a good inheritance fund to hand over to our only and I definitely don't want DC caring for us in old age. I hope to be like my maternal grandmother, she lived to 90 and was still driving herself around and living independently.
You cannot guarantee siblings will support in later life - many move miles away. That certainly has happened a LOT in my family as my parents moved from Southern Hemisphere to UK (leaving behind their parents) and my sister lives far far away from my mum and was no help when our father passed.