The US, and similarly many other western countries, hasn't been at the replacement fertility rate since ~1970 [1]. Relative to the years previous, its been pretty stable, but still not at the replacement rate: around 1.8, versus 2.1-2.2.

That's 50 years; a decent chunk of time.

By the way; its important to note that the replacement fertility rate changes over time. Today, in the US, infant mortality rates are half that in 1970 and prior, in basically every category. Ideally, obviously, the replacement rate will approach 2.0.

Is there a version of economic thought that doesn't necessitate constant population growth? How about the one we're living? You can make the argument that the world is falling apart, or the economy, or we're at the end of a totally normal 80-100 year economic cycle, or maybe everything is fine, that's all supposition. What's real is; we're still going.

The short answer to your question is: historically: No. Empires fall when population growth stalls. When empires fall, a LOT of people die, and a lot more suffer. The dream of "isolated sustainable communes" doesn't work at scale. If your argument is some variation of: the human population shouldn't be at scale; then feel free to remove yourself from the equation as a start down that path, but realistically, we're all here, we have the system we have.

However, I tend to believe that the better argument is: we found that version of economic thought which can work in the face of stagnating population growth. Its the one we have; it does work, not perfectly, in fact quite poorly, but we'll never know how bad the alternate realities are. In effect, MMT.

Its extremely and critically important to recognize that technology has jumped more in the last thirty years than in the thousands of years prior. Our relationship to it, as individuals and as a society, is still evolving, and that evolution is happening in the face of declining western populations. Technology amplifies individual productivity; so why are we all still working 40 hour weeks? Momentum primarily, but more specifically: our population only recently started plateauing. We needed technology to keep up; its the only way. But as population plateaus, technology will naturally take over more. Its not just "fewer people can do more"; it goes from "fewer people can do more for more people" to "fewer people can do more for fewer people", which naturally means they have to do less.

MMT plays into that because: technology is expensive. If we're entering a world where our options are: build a $50B water barrier around Miami or let the city drown, we can't afford to worry about the status of our gold reserves, or whether the blockchain is up today. We wouldn't have the money; we'd chose not to build it; billions of dollars in infrastructure, land, and people would be destroyed; and the economy would be worse off, not better, than if we had just said "blank check do it" and then dealt with the ramifications of that decision tomorrow.

That doesn't mean there won't be ramifications. There always are in complex systems. But people get so focused on the reality we're living, to say "if only we'd have stuck to the way we used to do it", without recognizing that its just as possible you, and all your friends, and all your friends' friends, would not be alive if we had stuck to that old system.

Here's the biggest kink though; generally, well-implemented MMT (which much like well-implemented communism, hasn't been done, but has been written about to great lengths) requires close coordination between monetary and legislative policy; something our government is engineered to not do. We're seeing the impacts of this miscoordination now; there's a strong argument that the biggest source of inflation today has less to with money printing or interest rates, and more to do with core productivity in the sectors experiencing that inflation (energy: oil processing, housing: home building, public transit construction, etc). Core productivity (more factories, more construction, automation, etc) can oftentimes be solved with cheap, targeted money; but the Fed is currently engaged in a show of raising interest rates. Its the one lever they have; every problem is a screw, they have their directive: a hammer. I don't feel this is doom and gloom, today; a lot of the issues we're experiencing are, truly, just the delayed echoes of COVID which will die out, raising interest rates will legitimately help some sources of inflation, and the legislature has demonstrated interest in providing cheap money for productivity improvements (build back better). But, its still how the Fed operates; they have their doctrine and their lever, and its easy to imagine a future where more coordination is necessary to push through massive spending in response to major issues like the climate crisis, declining water supplies, etc, without either succumbing to the crisis, or creating a new economic crisis.

[1] https://wtfhappenedin1971.com/