Funnily enough, infinite economic growth is just not possible physically and we're barely starting to understand it, although many are still in denial. That doesn't make any sense.
Not even infinite. Just having 15% growth year on year for 50 years would be consuming 1000x more resources in year 50 compared to year 1. It wouldn't take long to completely drain the Earth of energy and raw materials. (Which is partly why climate change is such an issue. The future outlook is much worse than past history.)
Fortunately, sustainability has become a major business priority for industry, and so the future might not be quite so bleak.
Playing with demographics like this doesn't work. Even just population stability is a very subtle equilibrium that is easy to break once we start messing with it.
That's precisely, and for a large part fast economic growth that we have experienced for the past 2 centuries, in particular in the West, that has led to birthrate decline. All "rich" countries have experienced it. Basically "importing" people from "poor" countries *to make up for it* is fundamentally absurd, and socially very problematic.
It might well be argued that corporate greed and wage stagnation has made jobs unappealing to the local workforce, and a migrant workforce has been taking up the slack. On the other hand, what has the local workforce been doing instead of taking up those jobs? Presumably they are working
somewhere? And if you took them away from those other jobs, what gap would it leave?
Speaking practically, anyone who has had dealings with the National Health Service in the UK recently will realize that if you took away all the migrant workers it couldn't function at any reasonable level.