(First, "We are not consuming more resources than the Earth can provide", is a false statement, as right now every year we (the whole Humanity) consume more than what the Earth can regenerate. It's an established fact. See
https://en.wikipedia.org/wiki/Earth_Overshoot_Day)
Nah, it's both, not either, of these elements of the solution.
Only lowering our way or life (in your scenario, lowering it at an unimaginable level), while being twice as many, is unacceptable, as we'll deal with absence or regression of medicine (too costly in energy or resources), drastic reduction or extinction of research and exploration, and end of the "global village" concept because of too expensive communication and transport. Education is at stake too.
And only reducing how many we are is as unacceptable, as it implies too much lives lost too suddenly, of course.
It must be a combination of both.
So, the point I raised stays true, we need (also) to reduce the population. But not only.
Right now I'm thinking I found the right way to explain the problem: even if you (and me, and a handful of other groups of humans) would agree to live like ancestral tribes, it would still not be the case of a lot, the vast majority, of people. And they would be much, much angrier of this plan, than of some alternative plan, e.g. to reduce the population and get altogether to a moderately poor level of life (not as drastically poor as if we keep on procreating. Say, way poorer than US citizens, but still wealthier than the average inhabitant of a Third World country).
And how to deal with social unrest is crucial, in how we'll solve the equation.