With or without population growth millions of people are going to die, put they are not going to die because of population growth.
People die every day all over this planet for various reasons and that is not likely to stop any time soon.
FDR made speeches is 1938, calling for a halt in exports of the precious commodity (fortunately, by 1938 the FDR speeches were well over their peak as hot commodity, and nobody paid much attention).
The current estimate of phosphate rock (and those estimates always go up) is in the region of 70 billion tons - enough to last for centuries, even if the fertilizer use stays in the current mode (which involves huge amounts of waste) and the population keeps growing at the rates of the previous century - and neither is even a possibility. And we are talking only about the immediately useful apatite-rich stuff. Phosphorus as such is an element that doesn't disappear - it is perfectly recyclable.
I guess, some countries may begin worrying at some point that 85% of the world's phosphate rock is located in just three places: China, Morocco, and USA. Well, be nice to us.
The industrial revolution has been unique in that no ceiling has been hit yet, and while we may not he bacteria in a Petri dish, we're still not omniscient enough to know what will present the next ceiling and avoid it. You'd have to be a betting man to predict unstoppable/exponential growth.
Your Malthusian friends solution is pretty stupid though.
"And in the end, we were all just humans, drunk on the idea that love, only love, could heal our brokenness."
I don't think overpopulation will lead to a mass extinction of man but it sure will ,effect his quality of life.