(This is not an argument against supporting parents. I just don't understand the concern around birth rates.)
Even if you can convince educated, relatively younger population of poorer countries to come and work for you, integration is a real issue. As an expat who lives in a rich country, I can say that; it's not easy. But if most of the native population turn into seniors, especially if they start to feel like a minority(age/ethnicity/cultural gap), I think that would be very annoying for them.
Not enough people to take care of seniors? That's not really a problem. The problem is it pays better to do other work that's useless or downright destructive to society, like trying to show users more ads to get them to consume more.
[citation needed]
> Not enough people to take care of seniors? That's not really a problem.
Unless you're a senior or care about your parents...
Edit: to be clear, I see possibility of selfishness as a side-effect of being childfree, not a corelation.
Some demographics certainly have higher ratios of unpleasant people. In some demographics it's even close to 100% (e.g. neo-Nazis).
It's entirely unclear to me if the rates are different for people without children. If they are, then you really need to do better than this kind of armchair analysis that you've posted here.
but there are a lot more pensioners and generally people without kids who prefer to vacation away from other people's screaming kids