I was thrown off by your “replace”, replace who? Maybe you were thinking about a hypothetical perfect mathematical world that started off with a lot of couples in the 1st generation and people died after giving birth.
The context though is the current real world: It’s perfectly fine to have 1 kid or even no kids at all and just adopt existing orphans, because a lot of other people have been having way more than 2 kids for way too long.
The context was the ethics evaluation tool of asking "what if everyone did this?" implying, if everyone can't do a thing, then maybe it's not right for one to do a thing.
Someone misunderstood the purpose off that question, and said that by that logic everyone should have 2 kids. They were wrong about that but not because the number 2 was wwrong. 2, or really 2.something, is the number that everyone has to have, IF they were applying the "what if everyone does X" correctly.
What they were wrong about was a couple things:
1 The question isn't a simple litteral absolute rule. It's a tool that you use along with others to evaluate some idea or action. It only helps judge one dimension among many.
A previous commenter gave the simple example of queues. Imagine you have figured out a trick that lets you get away with jumping every line. The trick works. No one else in the line ever complains that you're cutting instead of waiting for your turn.
What's the harm then? Free market people like to try to excuse bad actions that way. Maybe the trick is you tell a joke, so it's a transaction, the people you cut in front of voluntarily let you and they got a funny joke out of it.
The question exposes the problem, because, ok, cool, life hack, do this trick and never have to wait in a line again. Since it's sound advice, let's teach this knowledge to everyone in schools. What happens then? Does it still work?
This exposes that this advantage you are taking is actually coming from somewhere. You aren't just more efficient at lines, you are benefitting only on the backs of everyone else.
2 In this case the X in "What if everyone did X?" wouldn't be "have exactly 2.36 children" it would be more like "had however many children they wanted"
Because it turns out that people want different things, and so if you just let everyone have however many kids they want or feel they can manage, it more or less works out naturally.
Everyone doesn't want 5 kids, or 0 kids, and so if everyone did what they want, it's fine, and so it's fine for you to have however many kids you want.
You CAN in fact have just 1 kid if you want.
But the thing you were replying to was about the basic math, and 2 was the right number in the context of that oversimplified misunderstanding of "what if everyone did that?" If letting people do what they want would not work out, and we had to make a rule to make everyone do exactly the same thing, it would indeed be everyone has to have 2 kids.
The context though is the current real world: It’s perfectly fine to have 1 kid or even no kids at all and just adopt existing orphans, because a lot of other people have been having way more than 2 kids for way too long.