I remember reading the book when I was a junior in engineering. If I recall correctly (it’s been a LONG time), but book actually was careful to not spell out exact dates, but general trends. The horizon for collapse could have been as short as 50 years but as long as several hundred. The thesis was ultimately rejected because it didn’t take sufficient account of changing technology to control pollution, improve crop yields, etc.
But, it is probably correct over the long run, say several centuries or maybe even millennia. The earth sustains 7 Billion people today. Can it sustain 15 Billion? 25 Billion? 100 Billion? 200 Billion? 500 Billion?
At some point, you do run into the “carrying capacity” of the planet — arable land is finite, the earth’s crust is only so thick and endowed with only so many minerals, and the atmosphere only a few miles deep.
Should we spend any time, energy and money worrying about such things today?