I think you can have a high quality of life even as a child with a sibling. There are good things about being the only child, I was one. I had all the attention from my parents, yet, they didn't really give me that much of it. I was mostly left to myself and played quite well with my toys and in my imaginary world but they hardly ever read to me or talked to me. They were busy with their adult lives and I was just tagged along. I had a lot of friends, though.
Now, from a perspective of a mother, I have two and I for sure give each of them more personal attention than I ever received as an only child. So this really depends on your family dynamics. I spend all my time with my daughters and take them places, talk to them and read books. They for sure have plenty of mama time. They also have each other to play with and share some intense moments. They can also learn at home something I never did, how to negotiate and have arguments with other people and make up for for it later. They learn how to be self-confident and compassionate because they have to work out through some deep emotions together. I never had that. I never had to share my toy or give up anything and I never had someone younger look in my eyes with such love and devotion.