Actually, the growth of our energy use is intrinsically unsustainable https://dothemath.ucsd.edu/2011/07/galactic-scale-energy/
As to the technological ability to substitute fossils with biofuels, sorry: https://www.annualreviews.org/doi/10.1146/annurev-environ-121912-094620
As to the technological abiltity to substitute fossils with renewable electricity, there are some very serious problems there https://escholarship.org/uc/item/9js5291m#page=182
The fossil energy is no longer terribly cheap or abundant, if you look at the numbers. Per capita energy use has been declining, net energy per capita (the only relevant metric) has been quickly declining, and we are about to reach even volume decline perhaps as quickly as 2025, since the global liquid increase is almost completely due to Permian tight oil extraction alone. There are no longer any accessible fossil organics at scale left after tight oil and gas.
As to millions people perishing, the unfortunate suggestions is that it will be billions, though hopefully over a period of a century due to increased mortality and subreplacement birth rate, rather than one fell swoop of nuclear war and famine.
You are assuming exponential growth forever, where obviously in such a case within ~1500 years we will be using more energy then our sun can provide which is absurd assuming we stay on the earth forever. Plus you are getting to fanciful concepts like type 1 civilizations. Let’s get back to earth and say that TODAY at our current energy consumption, we can replace 1 for 1 our current fossil fuel energy budget with something else. But we don’t because TODAY oil is abundant and cheap.
If you want to talk about the idiotic Capitalist assumption of infinite growth forever, obviously that is going to stop, it has to. So when it slows (and assuming nothing else changes) millions will die as the quest for profit continues. None of the above is inevitable. I maintain a government that exists for the good of the many people instead of profit for a few at the expense of everything else, can absolutely become sustainable. There isn’t even a lot that would have to change for the average person!
As for the doomsaying about 2025 volume decline, I heard literally the same argument with peak oil in the 2000. It’s a misleading number that assumes we can’t replace oil with literally anything else, even though we and have been able to since the 50’s.
