How about arable land (able to support growing crops) since it is a major deciding factor on how many people a country can reliably sustain over time? It is also one of many major factors. Example, India has a little over 50% of it's land is arable, where the US is just under 17%.. Now that is also skewed because it's based on percentage of country total land. The US actually has more acreage that is arable land than India. But then you have to get into ease of access, ability to farm the land (equipment, man power etc).. there are many factors that go into determining how many people a country can sustain. Population density does not determine that.. it is the product or results of those factors.What would you use then? It would certainly be a problem if they all came at once, but as long as they trickle in a few thousand at a time we can scale up our economy with them.
More mouths = more hands = more work = more prosperity.
That is how an economy should work.