When someone says something about only children being this or that, I always remind them that I know just as many selfish, annoying or anything-else-strange from multiple-sibling families as I do "only kids with problems".
It's not the number of children, it's the parenting. It's the opportunities the parents offer, the amount of time the parent is willing to spend with the child, and if the parent spoils the child (because of guilt, because they're the apple of mommy and daddy's eye, because they don't want to actually engage--- endless reasons). Parents who have larger families and aren't tuned into their kids run just as much risk of having children who grow into adults with emotional problems as a single child with disconnected parents does.
My son is an only. We didn't have only one to spite him. :) We knew, honestly, how much we had to give: of our energy, our selves, our finances. If we wanted more kids, I'd have to go back to work. We try to live what we value. He gets a LOT of us. He gets great time with friends and peers (not just at school) and he gets a lot of love and special one on one time with us. He doesn't have a brother or sister to play with-- that's true-- and he also doesn't have the sibling bickering, sharing space, sharing the parents' affection and all the other insecurities that go with having multiple children. He knows how to share because it's been taught from an early age. Because we model what we want to see from him.
As for 'what about the parents dying? they'll be all alone..." -- well, we are raising our son to be a loving person, capable of forging relationships in this world all on his own. Being a person who has really had to start over in some aspects of my life, I'll say this: we choose to love, to seek others-- or not. As long as I am doing my best to create a stable and loving home for our son, I cannot also be responsible for his choices he makes when he's an adult. Even in a perfect world, even if I do every single thing right as a parent, I cannot control this. This is where we, as parents, have to let go and realize that our children are indeed their own persons. We lay the foundation for their future, true, but they must choose to build on that foundation in good ways-- we can't do it for them.