I will answer on behalf of two only children I am/was close to. My mom, who died just a few weeks ago, was an only child. Yes, as an adult, she always wished for a sibling. It was in part for the closeness of family and in part to help share caring for her parents. Her mother is still alive at age 90. You can't even imagine the heartbreak she has gone through burying her only child. I was the one who told her my mom had died. It was heartwrenching. My mom was close to a variety of cousins as a child and teenager, but as adults they either grew a part or moved away.
My husband is also an only child. He never really wished for siblings as a child, but I think he would find them useful now. Like my mom was, he is left alone to care for/entertain, etc. his mom and she can be challenging at times. He was raised by his mom and stepdad with some help from his grandparents. My husband never met his biological father until he was 30 years old. They had become close in recent years and then his father died in June. As an adult my husband met three 1/2 siblings he never knew he had as a child. They are now developing relationships and became closer during their father's illness. However, the four of them are scattered throughout the country and it's not the same as growing up with siblings.
We have two children so my husband did not repeat the only child experience. Ironically, he was the one who adamantly wanted to stop at two, but now wishes we had had more. Too late. (-: Unfortunately, our two boys don't get along that well, but I'm hoping that will improve as they get older. They are teenagers now and have very different interests. They do bond and hang out together when we go on vacation. Before we had our second son someone once asked me if we were going to have another and I said yes, in part because of the experience of my mom and my husband with their mothers. So then I realized I wanted my son to have a sibling in part to save him from me??????? We wanted another child regardless so that was only a small consideration.
Both my mom and my husband were/are very caring, generous, well-grounded people. Being raised as only children did not have a negative effect on their outcome or success in life. They did, however, wish for siblings as adults, especially my mom, more so than my husband.