I'm chuffed for you and your lovely children, (my 2 are really close too,) but quite honestly, it's not like that for many people!
I don't think 'only children' are at any disavantage, except the care of the parents will fall to them and them alone when the parents are elderly. May not happen of course as not all people end up infirm and needing care, and many daughters (especially the eldest,) will be pushed into that caring role anyway. Even if there are multiple siblings.
Having more money and time (by having just one) can be advantageous too. Many only children - like some firstborns will do better academically, and often will do better in their career too.
First-born children (and only children) often have better academic performances, and greater financial success due to increased parental attention and resources early in life. They tend to possess strong leadership skills, emotional stability, and a higher likelihood of achieving advanced education. Not all of course, but some. My second born is as successful and happy as my firstborn, and just as happy. But there is a tiny gap between them (less than a year and a half...)
It is not 'unkind' to have just one, and there is NO guarantee that if you have a second child that they will get on. I know more instances where siblings don't get on than I do where they DO get on!