It is really difficult to make predictions x years ahead of most topics, the further the harder when you take into account the dynamism of these factors and the innovations (discoveries on how to use new materials, for example, that replace old ones, etc.)
It's as you said:
> It is really difficult to make predictions x years ahead of [things like] innovations
Humans already use more resources than they regenerate (the definition of unsustainable). Currently, most of the world population lives in relative poverty compared to the people in places like Germany, USA, Japan, etc. It would be fair to have everyone live at this standard, but that costs even more resources, making it even less sustainable! We can hardly prevent other countries from wanting to innovate, so it will happen, and our problem will get worse if we do not gain better technology (and/or reduce our consumption of animal products, learn to stop heating offices across weekends, etc. It works on both the supply and demand side of resources).
Precisely because it is hard to predict whether future innovations will materialize soon enough to fix those problems in a way that we do not reduce everyone's living standards, we cannot rely on the innovations happening and being deployed earth-wide in time
Even if this was true, recycling can exist. Also, other materials can replace older ones. Given these, I am not sure what the right strategy would be, unless it is really urgent and justified (I always hear a lot of generic alarms but no concrete facts that something is going to happen AND it happens). Take into account that, especially for developing countries, resources and cheap energy are critical for development.
No matter how much effort a spanish does, for example, in the reduction of CO2 (given the models are correct and that it is humans who warm up The Earth, something I am not convinced of at all), we are around 0.8% of total emissions. On the other hand, preventing China from burning coal was (last time I checked) making 400 million people back into poverty.
Do you see the problem? The problem could be ecological to some extent. But... are we controlling investment vs return? Are models true? For me it is impossible and I think there has been a strong bias towards exaggerating because there is a lot of money in the middle, independently of the truth.
That is my current view. They have been announcing the apocalypse since the 90s: everything flooded, global warming (that they conveniently changed to "climate change")... a lot of weird stuff.
A highly politicized area of discussion and a huge business for some.
Negative slope population will lead to mass economic instability and eventually war.
A USA with 70 million people would feel very threatened by an Iraq with 300 million, for example. Just as Russia feels very threatened by Europe now.
Economic problems are real and worth addressing and I hope we can find ways other than “we need more babies”.
I believe that in case of population collapse, the solution will naturally appear to large enough number of people, evolution shaped us to reproduce, I don't think this instinct is likely to disappear, even if we might not fully understand it and why girls and women tend to have less children in some conditions (it happened before, in antiquity, and was considered to be a problem by Roman politicians, many of the current traditions around marriages were first created as laws...)
The solution evolution found was to make us really like having sex. But then we invented hormonal birth control, and sex became uncoupled from reproduction. Evolution can't help us here, it works slowly over thousands of years, and can't uninvent birth control.
This isn't always the case; evolution can happen relatively quickly in response to rapid changes in the environment [1]. There are plenty of people alive today who have particularly strong urges to have children, and the widespread introduction of birth control creates a major selective pressure towards any heritable genes that produce or enhance those urges. The effects of this could be very significant over even a relatively small number of generations.
Housing policy, tax policy, education policy, healthcare policy, family law - governments are influencing population massively.
I think there is ample buffer for future generations to correct the population trajectory if under-population happens to be a non-imaginary issue for our species.
In my opinion, many ecological problems would be simpler to solve with half or a quarter of the current population.
And the imbalance between old and young would only last for a few decades.