Reading the study I get the following remarks:

Living space, not great. 60m2 for a 4 person family. That's tight. I live alone in a 90m2 house and I could use more space, do they want me to live in a 15m2 house or do they want to force to share living space? Sorry but I won't compromise there. I prefer people having less children that me having to live as ants in a colony.

That is just a personal pick with the DLS minimum requirements chosen.

But still forgetting that. The reasoning is extremely faulty. Most of their argumentation heavy lifting is just relied to Millward-Hopkins (2022) paper establishing that 14.7 GJ per person anually is enough. That paper is just a work of fantasy. For reference, and taking the same paper numbers. Current energy usage (with all the exiting poverty) is 80 GJ/cap. Paleolitic use of energy was 5 GJ. Author is proposing that we could live ok with just triple paleolitic energy. That paper just oversees a lot of what people need to live in a function society to get completely irrational numbers on what energy cap we could assume to produce a good life.

Then on materials used. The paper assumes all the world shifting to vegetarian diet, everyone living on multiresidential buildings, somehow wood as the main building material (I don't know how they even reconcile that with multiresidential buildings...). And half of cars usage shifting to public transport How to achieve this in rural areas it's not mentioned at all).

A big notice needs to be done that both papers what are actually doing is basically taking China economy (greatly praised in the introduction) and assuming that all the world should live like that. And yes, probably the world could have 30 billion inhabitants if we accept to be all like China, who would we export to achieve that economic model if we all have a export based economy? who knows, probably the martians. And even then, while a lot of "ticks" on what a decent level of life quality apparently seems to be ticked, many people in western countries would not consider that quality life, but a very restrictive and deprived life standard.

I'm still on the boat the people having less children is a better approach to great lives without destroying the planet. At some point a cap on world population need to be made, it really add that much that the cap is 30 billion instead of maybe 5 billion? It's certainly not a cap in the number of social iterations a person can have, but numbers give for plenty of friends. And at the end it's not even a cap on "how many children" can people have, as once the cap is reach the number of children will be needed to cap the same to achieve stability. It's just a cap on "when people can still be having lots of kids". Boomer approach to "let's have children now" and expect that my kids won't want to have as many children as I have now.

Also another big pick I have with the article is that it blames the current level of inefficiency to private jets, suvs, and industrial meat. But instead of making the rational approach of taking thise appart from the current economy and calculate what the results will be. Parts from zero building the requirements out of their list. Making the previous complaint about those luxury items out of place completely. On a personal note I would reduce or completely eliminate many of those listed "super luxury" items. But I have the feel (just a feel because neither me not the author have studied this) that the results of global energy and material usage won't drop that much, certainly not at the levels proposed by the authors with their approach.