DH and DD are both "onlies", and very happy with that. I was one of four, and that was fine too.
End of life care: thanks for the reminder - we'll be making sure that we have enough provision to make it as easy for DD as possible - we'll try to avoid being a burden. As others have said, the burden often falls on one of the offspring anyway, and there's sometimes a lot of tensions between siblings at that point (how many threads have you seen on here...?). With families often very widely flung, elderly people can often only live within range of one of their offspring. At least the decision of which will be easier.
Characteristics of only children: I'm sure there are some that are spoilt, but PLEASE don't make those generalisations, because it's very unfair on those who are not. I think the biggest thing we've noticed is that DD used to be very easily upset by the sort of insults siblings fling at each other all the time - she took them much more seriously than they were intended, just because she wasn't used to them. Apart from that, if anyone had criticised her social skills whilst at primary, and tried to put it down to her being an only, I would have had to point out that I was very similar, and one of four, so it was probably just hereditary.
I do wish my DM had kept her opinions on only children to herself. When weighing up whether to have a second, I eventually realised that the biggest reason I wanted a second was her strong opinions on onlies. To be fair, she didn't voice those opinions at any point after we were expecting our first, but that was little use - I could remember what she'd said in the past. So remember, when you say things you wouldn't say in front of parents of an only child, that saying them in front of people close to you who may be future parents could be just as hurtful.
I have been fascinated that DD's friendship groups have had a high proportion of onlies - in primary they were all onlies or significantly older siblings. Some of that may have been differences in how they related to people, as they were all used to more adult-dominated conversation at home. However they were also all similarly bright, and had lots in common other than being onlies - their mums also had lots in common.