The planet is already home to 8 billion human beings and we can't afford to increase that number by much.
Malthusianism is a dead theory, the world was supposed to be overpopulated at 1 billion, 2 billion, 5 billion, and look how we're still growing and there is no such thing as overpopulation, there are few countries with problems when it comes to food supplies but that's not because of the planet, it's because their own choices from the uncontrolled birth rate when they don't have enough food even for themselves to the total idiocracy of their governments.
But excluding those poor regions of Africa there is no sign of overpopulation anywhere, maybe just some extreme cases when we're talking about islands like Singapore
For economic growth, why there is a requirement of constant population growth? Why can't it be achieved with a stable population? Countries such as Germany and Japan are having stable populations, and still their economy is doing good.
Because you simply can't.
For economic growth with the same population, the one replacing the poeple in retirement would have to both produce more and consume more, as those products or services need clients too. So at one point, you will hit a wall, it's pretty simple, just think of how much could you consume and buy.
Let's assume you don't have problems with money, you buy 100 cars, but can you drive 10 of them at the same time ? You can afford the best hotels in the world but will you book 700 nights in 10 hotels in one year?
Can you drink 100 beers each day? Or go to a concert, cinema, aquarium, gliding, skiing all on the same day?
There is a limit to how much a human being can consume even if it throws away stuff, when this limit is reached and there is no increasing demand from population growth then you hit a wall, as extra products or services aren't needed, so goodbye growth.
Of course, for a period you can still achieve growth even with a stagnating population till to manage to cover all the needs of everyone,but sooner or later it will come to a stop.
And there is a second problem, with a declining population and a smaller workforce but a higher percentage of retired poeple you have a smaller taxable population with an increased burden on social welfare costs, and this is the main problem for Japan for example.
With nearly one-third of the population being older than 65, 15% unemployable because of their young age, and this getting worse and worse how will you get the money to run a country and from whom?