I am an adult only child and I do (unfortunately) agree and relate to the concerns you have for your daughter.
Of course siblings are no guarantee of a close relationship (especially in adulthood) but like you, amongst friends and wider family I do generally observe supportive relationships.
I am thinking of an aunty for example. She has four sisters. When one of those sisters was diagnosed with cancer in her 50s, the other sisters rallied round and took great care of her. The sister with cancer is single and would otherwise have been quite alone, I think.
I think only children can be quite resilient however. I would love a close in-built support network, but I know that isn't possible, so I work hard at trying to be a good friend and hopefully someone people value and appreciate the company and support of.
Also (not a given I know) but only children are sometimes better off financially (through not sharing inheritance etc) so are better places to be able to afford professional support services.
Nieces and nephews, yes unfortunately we miss out on them (unless through marriage). However I am not sure non-blood related nieces and nephews are quite the same.
If I had my life again and could choose my family set up, I'd like one or two sisters who I'd have a close relationship with. But of course we can't choose and we have to make the best of what we have - which I am sure your daughter will do too.