I'm an only and had a lovely childhood. My parents are old now and, going by statistics, will die in the next few years. I have one cousin whom I haven't seen in 20 years and the rest of my family (grandparents, aunts, uncles) are long gone so I'll be on my own. I'm fine with it. Of course, that would have been a different story had I been 10 but I'm 40. Yes, it'll be down to me to look after them if they need to - they are adamant that they don't want me to and have made sure they have enough savings to cover so many years of care, but I would want to support them as best as I can.
My husband has a brother. They get along fine but can go for long stretches of time without seeing each other. I can't say I look at their relationship and think I've missed out on anything past, present or future. My husband is already doing the bulk of the caring so I doubt his brother will have an epiphany any time soon.
Some may resent their parents for being an old child, but there will be as many resenting having siblings. I'm relatively sure that when parents stopped at 1, there was a genuine reason be it financial, health related, choice based on experience. I very much doubt anyone decided not to have another child out of spite to their existing child. Of course, said child may end up unhappy with their life, but there'll equally be people who will wish they didn't have a sibling.
I think there are a lot of stereotypes about only children here in the UK which I didn't experience when I lived abroad, and I feel this feeds a sense of unease and guilt amongst parents. In this day and age, we should all know families come in all sizes and shapes, and this is fine.
My husband and I didn't have any set idea about how many children we would want and we'd said anything between 0 and 3. At some point, we decided to go for one and see how it went. Let's say we don't want anymore, and this is fine. Having a second child when we are not in a good place to would do more harm than good.