Any political system can end up in fascism or despotism. Some have argued that the capitalist west is essentially a fascism of desires, images and consumables, but nobody notices because we're all too busy buying things. So the argument that governmental interference in population will lead to fascism just doesn't go anywhere. Of course it can, but so could any system. The argument is a pointless tautology. No doubt people have used population control in the past to further ulterior agendas. Eugenics mostly came from the left and also sprouted environmentalism. But that doesn't mean that these things have evil eugenic goals today. American popular music was once permeated with racial stereotypes (and still is to some degree) but does that mean we should call all popular music today racist? The socialism=fascism argument has the same form.
Though no one really knows for certain how to measure the earth's human carrying capacity, warning signs abound that we're either close to it or way over it. Some look to technology for guidance, but that's never a guarantee (Green Revolution crop yields have dropped dramatically since the 1970s). But even if another famine-busting technology emerges, we'll still face limits. The earth is a closed system that can't accommodate limitless growth. No current system can.
Good news exists in plummeting global birthrates. Well, good news in one sense. Some demographers argue that as we reach peak population we're also headed for a population crash in a few generations. People are having fewer children today in general, but they may be having too fewer. Decreasing population cascades similar to increasing population. But maybe this is just what we need?
In any case, the tendency for population talk to degenerate into analyzing human beings purely in terms of statistics is too real to ignore. If we value life we can't just say "those people over there are worth less than those people over here." We should have learned from the 20th century that such thinking can lead to catastrophes. Also, from a purely selfish perspective (the only perspective many myopic people can identify with) if you don't value people "over there" they may not value you "over here" and if the people "over there" hold the power cards, you may find yourself in the midst of a nasty population reduction program. People are people no matter how tangential they are to our own narrow concerns. The challenge of the next century will be to de-program the selfishness that seems to now permeate the west. We're really self-absorbed. It wasn't always this way.