This is more of an abstract answer but there's a sort of existential comfort in never being alone after having a child. In my 20s I was at uni in London and walked around a lot by myself, and also lived alone. I had good friends and relationships but everything just felt so temporary in that phase of life. Wandering around a big city alone often gave me a sense of deep loneliness. I wasn't depressed or unhappy but it was a feeling of melancholy. I felt adrift like everyone's lives were unfolding and passing me by. No matter how lovely my friends, experiences or memories were, there was nothing left once that event is over.
I often replay those memories, but in my head I put DD in there like a small invisible person from the future (angel sounds a bit too corny since I'm an atheist). It gives me huge comfort knowing that all the times I was wandering around the streets by myself, or waiting in rain for a bus that was taking ages to come, there was my child from the future next to me and I never knew it. Even though I was lonely at the time, I didn't know how important I would be to another person someday.
I'm scared by the thought of growing very old, and eventually my parents and husband won't be around either, and walking the streets alone just like I did before. Except this time it won't be the melancholy longing of a young person who hasn't found their niche in life yet. It's basically staring into the void, knowing you are alone, your body is failing and there is nobody else around. I can't imagine much comfort derived from spending the last phase of your life alone, tired and confused in a nursing home with lovely people, but who are effectively total strangers. Every day would feel like lockdown does right now, except with visits from children or grandchildren to break up the weeks and months. There would be nobody who genuinely cares about your health should anything happen, and you are left going to medical appointments alone as well.
Obviously, this is a personal thing and I imagine this scenario doesn't bother plenty of people at all in exchange for the freedom of a childfree life. Big advantage of not having kids is being able to spend time on yourself, and knowing you can truly do anything you want for yourself. The faff of taking care of kids is stressful, it only hit me recently how much extra work is involved shopping, cooking and feeding a family compared to one person. There is no break unless you get ill, and even leaving for a few days requires massive preparation beforehand.