IMO this apparent wish for siblings has more to do with a romanticised view of what having siblings is said to be like based on a societal expectation.
How many times do people ask a child “would you like a brother or sister?” Even in passing, the idea is planted in their head. If you didn’t put the idea in children’s heads that being an only child was somehow wrong nobody would ever think twice about wishing for these mythical siblings.
And most siblings don’t have close relationships, playing together as children doesn’t mean growing up with a special bond. Most siblings move away from each other, and while they may not hate each other most siblings have an acquaintanceship at best, the only reason why they even have that is because of expectation, because they’re siblings.
As for siblings being there for each other when the parents die, that’s all very well, but what if one of the siblings has SN which means they can’t be, should the parent keep having children until they end up with two who might be in a position to be there when the parents die?
Or what if one of the siblings dies before the parents? There are two people in my family alone who have lost a sibling meaning that they’ll be caring for parents alone.
OP nobody else’s opinion matters, because it is just that. Opinion.
Personally I think having three children is selfish because the middle one is generally left out, and I know several people who hated being a middle child, just as there will be people here who say they hated being an only child.
Nobody is right here, because everyone’s experience is diffferent. So you have to do what you feel is best not what a load of strangers on the internet tell you you should be doing based on what they think.