I know far more siblings who hate each other's guts than I do "lonely onlies".
I have an only, I am an only, my Dad is an only.
None of the 3 of us could be described as lonely.
How many siblings truly truly say (and mean) that their brother/sister is their best friend?
Siblings are handy to have around so you can get the hoovering done while they play nicely......if they play nicely. 
There are lonely, introverted people who are only children, there are lonely, introverted people who have 5 siblings.
If you want another anecdote (for that is all anyone can offer you, both the anti-onlies, and the pro-) (although I believe there are statistics that show only children are likely to be more intelligent,independent and able to mix better in adult company) dd, as I have said, is an only....is surrounded by a gaggle of friends, at ease in every social situation, never looked back when she started nursery, or school (t'was me that cried, not her) whilst her best friend (who has a younger brother) is a shy, timid little thing who cried for the entire first year of school,and is now, just, at 8.5 starting to dare speak to anyone who isn't her mother.
Because being shy/lonely whatever, is nothing to do with how many people are around sharing your DNA.
Btw, there is a dedicated section of MN for only children (parents of) The "tearoom" thread is one of theirs. You'll find lots of advice on there!
As for the looking after aged parents thing- again- how many siblings truly share the burden???? My Mum is one of 3, and when her mother had dementia, she did the wiping arses bit, while her brother and sister arrived on Mothers' Day and at Christmas with the biggest fuck off bunches of flowers they could find, for a person who no longer even knew what a flower was.
I'd also ask, genuinely, because it has never crossed my mind, as mother of an only and daughter of an only, why would anyone ever be thinking about "what will happen when I'm old and doddery" when they are planning their family? That hypothetical sibling might move to Australia, or be run over by a bus, or simply send one of those bouquets twice a year.