@WinteryWonderland I have no children, no siblings, and all my family are dead (mum 16 yrs ago, dad 2yrs, grandparents obviously longer). I'm early 40s.
Partner was married, which ended, and has 1DC & co-parents with his ex GF. He has both parents living and 3 siblings who have families, all pretty close. Most are nice, one not so much, but I wouldn't keep up a relationship with them if we weren't together, nor them with me - just not deep enough bonds or enough in common/shared history.
TBH, since I never wanted children, that doesn't figure in my mind much. I miss my grandmother a lot, but I don't really worry about being alone.
I have lots of lovely friends that I'm very close to, and who I'm inviting for Christmas - have done this since mum died, so it's my normal.
I'm quite in the "friends are the family we choose" camp, and even tonight, was lying on the sofa with DP, and thinking "you're older than me (not that much, but still), I wonder whether you'll die young (like my parents) & I'll have to do all this again".
But we never know what's coming round the corner, so I figure I may as well just enjoy it now and take what comes as it does.
And also keep doing classes & things that interest me, and asking people I meet at classes etc out for coffee if they seem like they might be interesting - making new friends is a useful skill, and developing it means that I'm not really afraid of being lonely, because I know I can find new people to get to know in whatever contexts.
I think, although I can totally understand why you might be afraid of future loneliness, the reality is that it could (& does) happen to many older people due to many different reasons, whether they have children, family etc or not, so working on the proactive steps you can take to counter it is maybe one way to reduce the fear.
I don't currently, but used to volunteer with older person befriending - I really enjoyed having tea and cakes with them and maybe getting perspectives from people in that situation might help a little as well - they were taking steps to meet new people and were mostly pretty upbeat, happy and enjoyed their lives, despite lack of family connections...