When I was single (so on and off through late-teens and twenties), the minuses were being on your own so much, I love my own company, but every weekend and every evening often got a lot to fill. I was also often out of step with the lives of my coupled up friends so there was a lot of being the only one at a dinner party. I solved this by moving somewhere there were lots of other single people too, and going out a lot, but it always seemed very unstable, as friends would leave work, or meet some new bloke or I'd date a lot and not meet anyone who quite fitted, you always knew that you simply weren't the most important person in anyone's life. And, for me, having a primary romantic relationship was very important in terms of personal fulfilment, I have only ever fallen in love once.
There were masses of pluses as well, I pleased myself, enjoyed my own company, had a higher standard of living, enjoyed lots of more casual relationships. I also preferred living day to day by myself and still do.
But there's a big difference between being single after a terrible marriage with your children to keep you company and the purpose of being a mother (when singledom may be a blessed relief) and not knowing if in your forties you will ever be a mother or ever find a companion again (fine if you like not having children/a partner, not so fine if you don't). I don't envy my single childless friends tbh, nothing to do with the social stigma. The ones in couples who have decided not to have children do very well in terms of companionship and enjoying their lives.
I don't see singledom as a thing to aspire to any more than I do having a relationship at any cost (which I have never done). But I wouldn't teach my children to avoid relationships, only bad ones.