This seems obvious right? but babies, toddlers and children are COMPLETELY different beings to one another. It's a bit annoying as just as you get the hang of the one hour rocking and blasting hoover noise into your babies face to get it to sleep then BAM here comes another baby with a morbid fear of hoovers.
Some of our concerns are longer-term. While we do okay financially, our jobs aren't super secure. Having one child keeps us nimble, and means that if we had to relocate to a more expensive city, we'd have a decent shot at maintaining our quality of life.
There are children who were raised as only children who were happy to be so, and others who were desperately miserable. Likewise, there are children raised with siblings, maybe just one or perhaps more, who absolutely loved it, and others who look back on their childhoods less enthusiastically. Only children, two children, ten children - there are advantages and disadvantages to each.