So much I want to say here...
I'm part of the choir to whom the author is preaching but I find this weak tea. (Although he's not preaching, he's RFC'ing his assumptions.)
If we grant that fusion isn't around the corner (and I don't, see "Fusion in a magnetically-shielded-grid inertial electrostatic confinement device" https://arxiv.org/abs/1510.01788 abstract: "Theory for a gridded inertial electrostatic confinement (IEC) fusion system is presented that shows a net energy gain is possible if the grid is magnetically shielded from ion impact. A simplified grid geometry is studied, consisting of two negatively-biased coaxial current-carrying rings, oriented such that their opposing magnetic fields produce a spindle cusp. Our analysis indicates that better than break-even performance is possible even in a deuterium-deuterium system at bench-top scales. The proposed device has the unusual property that it can avoid both the cusp losses of traditional magnetic fusion systems and the grid losses of traditional IEC configurations." So that's neat.)
If we grant that we must massively reduce our energy consumption that is not necessarily mean lower Q-of-L. Our systems are hugely wasteful (count the number of LEDs in your house tonight. A first-world household burns so much energy so pointlessly. The not-really-off nature of modern electronics is a small part of it. Bad insulation. Pilot lights. Refrigerators that open like cabinets rather than drawers. Incandescent "light" bulbs that put out more heat than light: they're literally heaters that also give off light. I could go on and on and on...)
Bottom line: if we took physics seriously and gave a shit about efficiency we could cut our emissions in half overnight without any major technology change in production. (Although we should still do that too.)
Also, ever since the 70's people have studied e.g. passive solar design for buildings. We can easily make our cities more efficient and more ecological with just a little cleverness. (Remember that time when we accidentally built a solar furnace in London: https://www.nbcnews.com/science/science-news/london-skyscrap... ) The power is there we just have to use it wisely. "It's raining soup, grab a bucket!"
And with applied ecology (e.g. "Permaculture", regenerative agriculture, urban "food forests", etc.) we should be able to recycle wastes and produce food in situ in urban and suburban areas in densities great enough to sustain Arcologies ( https://en.wikipedia.org/wiki/Arcology ) integrated ecological city-buildings that provide high quality-of-life in an ecologically harmonious and sustainable way. (The beginnings of this are already happening in parts of asia. ( E.g. https://www.quora.com/How-is-China-able-to-provide-enough-fo... ))