One of my children in particular is an extrovert and would struggle horribly as an only child,
Dd is an only child and fairly extrovert. As an only child this has benefited her hugely. Tbh it may also be one of the reasons she is confidently extrovert. Dh is too whereas I'm not. Dh both have siblings.
Dd has so many friends and is really happy as an only child. Though she is now a young adult at 20y. Being an only child hasnt held her back once and, tbh, being an only has obviously brought some benefits for her too - she’s had far more, opportunities available to her than if we'd have been funding two or more children growing up. She has lots of close friends, even more general friends and is very close to family members as well. She is confident, sociable, works hard at whatever she is doing, has empathy, is friendly, is happy to share/work with others, she never expects (despite being in a privileged position) and is kind. She doesn't fit the horrid and over-used 'only child' stereotype.
She has had periods of time when smaller where she’d have liked a sinking, but more time being totally happy without one. And now she says she is totally happy. She is generally far too busy with family and friends to be lovely. Besides she is also comfortable and happy to have time on her own too.
We could have thrown time, money and effort into trying to provide a sibling for her, but instead invested all that into providing dd with a wonderful enriching childhood and continue to have a fabulous time and relationship with her now she is older.
She has lots of friends who have siblings and is close friends with siblings, so it isn't like she doesn't know what sinking relationships see like - some very close, some very distant/almost non-existent and most for somewhere in between, and that it can change at different ages.
We can never really know what it would be like with or without a sibling, or what life would be like with no children, only child or several children if it's not what our current situation is.
Having a sibling can be wonderful but it can also be heart breaking and difficult. Sadly you don't know what version of a sibling relationship you will get until it happens.
We only know what we have and if that makes someone happy at that time - fabulous.