The problem is that all tigers want steak, but only a fourth of them can afford it. That number is increasing as the tigers get richer in general. And the steak industry is ruining the world.
The price of steak will go up until demand equals supply.
(And there will be plenty of incentives to come up with cheaper steaks or steak alternatives.)
That's the thing though, this price is going up but is artifically lower than alternatives because external costs (e.g. greenhouse gasses emitted or water used) are not factored in the cost.
I can definitely get behind some of the solutions they propose! But it is important to give a truthful rationale for these. Otherwise this is a slippery slope, like lying to your children about medicine because they will get better if they take it.
I stand by my assertion that the methodology they picked ("pick a subset of civilization, make a prediction as if everything was it") is fallacious. Imagine a sustainable, closed-loop civilization, satisfying all the goals you can dream of. It is inevitable that not every part of the system will be balanced when looked at in isolation. If you are allowed to cherry-pick a subset of that civilization and scale it arbitrarily, you are guaranteed to arrive at the same unreasonable conclusion as they give us.
To make it less abstract, let us consider nature itself. There are siberian tigers which eat 9 kg of food a day. This is a lot! If every animal were to eat that much, all animals would quickly starve. Clearly, nature is not sustainable!