I have one child, not by choice. My brother has 1 child, his wife’s decision after a difficult delivery. I now have 3 grandchildren, my brother has 2. Both of our children didn't want theirs to be ‘onlies’ but neither ever said they didn't like being an only themselves until they became parents.
There’s no hard and fast rule- my child and my brothers child wanted for nothing but weren’t spoilt. They are caring, compassionate, popular, sociable with many friends.
My brother and I are closer now than we’ve ever been. Certainly growing up, there’s 5 years between us, we had nothing in common and our early years were probably similar to those of an only child as obviously he had 5 years as one before I came along. Then as he started school I had my ‘only child almost’ experience.
There’s also no guarantee that siblings will like each other at all or be there to support and help each other during the tough times.
Do what you feel is best for all of you; there are many reasons for and against and I’ll always remember what my child's first headteacher said to me when I had a discussion with her asking similar questions to yours. She said ‘ an only child is not necessarily a lonely child’- which I think is a point made by many upthread.
Another school of thought is plan for 1 child and if another comes along later, well so be it, if not, well so be it.