Without borders would you not get people migrating to more well off countries, of course you would now if this continued for many years where would you house them.
Take Britain for example we already have a shortage of houses and yes we could build more but how many years could this go on for before we totally change Britain for ever.
I think our population is about 80 million ish I don’t believe over many years increasing the population to say 200 million would make it a better place to live.
Thanks mike