Not selfish at all. We have an only child, not through choice, we tried for a second but it just didn't happen (but we chose not to go down alternative routes, IVF, etc, so it was partially our choice!). We have absolutely no "useful" family, and are very low contact with siblings. When he was born, 2 of his grandparents who very elderly/ill so weren't really part of his life. His remaining grandmother spent DS's first years looking after her husband through protracted illnesses, so wasn't around much either. So, ultimately it's been me, OH and DS as a "family threesome" for virtually his entire childhood.
We made up for that as much as we could, by arranging play dates, taking him to clubs/sports etc to be around lots of other children, etc, being there for him ourselves, etc. He's turned out not as outgoing and social as his peers, still very quiet and perhaps a bit introverted, not helped by his first year at Uni being in lockdown, so he didn't meet people through the usual clubs/societies, parties, etc., nor even lectures (there were none!), but he's now in his 3rd year, living with 5 genuine Uni friends, so he's finally getting the "social" experience, and starts a good graduate program job in Summer with a big employer, so hopefully that will also bring him out of his shell even more. But by being an "only one", he's had so much more support/guidance from us, which probably helped him do better at school etc as we didn't have to spread ourselves around more than one and could help with homework, help broaden his interests/hobbies, etc.
Yes, he's virtually no family, and both me and OH have life limiting illnesses so we're not going to be around for long either, but, we've done all we can to cover for the lack of close family. You have to deal with the cards you're played. But close families aren't always all they're cracked up to be, both OH and myself had siblings but are now very low contact, as we're very different people to our brothers/sisters and have nothing in common, and didn't really live eachothers lives even as children, we'd do our own things with our own friends.!