We’re at 7.8 billion humans on this planet and rising. I don’t think we have to worry about the birth rate declining. If anything, we should do everything in our power to lower it as much as we can.
The data I've seen shows that poorer countries and families tend to have a higher birth rate than wealthier countries and families. To abstain from having children as a well educated, well off individual because you want the global population to drop is like not drinking a glass of water when you're thirsty because there's a drought, while farmers consume thousands of acre feet of water. It's not rational.
If the answer is, "well, the birth rate will never lower that far, because some people are going to keep having children" -- who will those people be? (Do you presume to be able to choose who gets to have children and who does not?)
Aren't those some people then doing a great service, keeping humanity alive? Shouldn't we value their efforts, and give them support?
Also, drastically lowering birth rates has negative effects on demographics and economies (e.g. a population pyramid heavily weighted toward the elderly, without enough young people to do the work of taking care of them, or the productive work of maintaining and improving society generally).