There’s a simple argument - the USA can obviously support some level of immigration - at the bare minimum the difference between current births and the replacement birth rate - and just as obviously it can’t stably take in half a billion people a year. Somewhere in between there must be a gradual cutoff where it becomes “too much”.

Most opponents of immigration say we’ve passed that mark and either need to compensate to solve the issues caused by it, or dial the number back.