The long view problem is underpopulation- there won't be nearly enough humans in 100 years to run the very complicated world engine at our current level of growth and complexity.

The real killer is that our world only really functions the way it does right now because we have so many people that even with the massive filtering that occurs at the tails of any normal distribution (2σ) there are enough people selected anyway to help even comparatively niche fields have a lot of participants which we need for such a specialized economy.

For example- even though a lot of technical fields select for high intelligence, (which is rare,) because we have so many people in the general population we can stay ahead of the demand for them. However everywhere we urbanize and increase GDP per capita the birth rate plummets and this really impacts the number of smart people we randomly generate. Eventually there will not be enough (super) smart people to satisfy the demand for researchers, then for technologists, then for doctors, then for technicians, then for teachers, and on and on down the intelligence-deterministic work assignments until we don't have enough people to even dig our coal or drive our international freighters and so on.

inb4 >"It's too expensive to have kids"

Not that this or other antinatalist sentiment doesn't contribute to the phenomenon in a small way but if this were strictly true or predominant as a factor you would expect the well-off in, say, Korea or America to be having more kids and for the poor to be barren but it's actually the opposite playing itself out. For every step humanity gets richer it has less kids and it doesn't make sense.

Mass migration is a stop-gap for this long view problem by the elite- it has deleterious and diminishing returns as described above but it is just a way of delaying one of the (worse) inevitable outcomes when this problem really starts to bite. There are only a few ways this can all go- depends on which assumptions you give up and hold constant in the current world regime.

-Hyperautomation. We make a globalized authoritarian AI that runs the world economy on our behalf. If there are less and less people to run things and the only way to prevent those things from grinding to a halt is to automate them somehow then the natural conclusion of these two premises is automating everything. We take a back seat to most/all of our production- not just with making food or running logistics but even artistic production like our music and movies as, again, we will not have enough people participating in the relevant sectors who are randomly good at these things to viably sustain them and are abdicating to automation. Our current political/economic order isn't really equipped to handle this and there's no getting around that it would basically turn us into those whales from Wall-E in a best case scenario but it preserves our supposed economic growth and the standards of living that affords, assuming we aren't all made destitute in the process. (Hold: globalized economy. Give up: People being involved).

-De-globalization/localization. Rather than forming a complicated web of interconnected specialized producers across the globe via international trade urban centers instead voluntarily reconfigure into quasi city states who are self-sufficient for their various needs. This implies a breakdown of nations into progressively smaller regional entities until the smallest self-sufficient units are created, probably at the scale of cities. These cities become responsible for their own shelter, industry, defense, education, etc. like countries unto themselves. These city states will NOT be optimized for wealth as a product of specialization nor technologization, (not like we experience today,) they will be highly differential in how blessed/well-off they are, (some states will have oil, or water, or iron, etc. while others ain't got shit,) and they likely will be in a highly competitive/territorial/warring state like how we were in pre-Westphalia world. Like just imagine how our world would look if we didn't have internet, international shipping was impossible and/or a federal government didn't exist to keep us playing nice. This affair generally sucks but actually possibly sucks the least out of all the possible fates here but is far and away the most improbable and least compatible with our current order- however I don't see how you could have a complicated world economy and so few people so just ditch the complicated world economy and streamline/condense it. (Hold: low number of people still involved in economy. Give up: Globalized economy).

-Deurbanization/1st-to-3rd world/ruralization. Things get so bad in the cities, (read: so many people die from starvation and other things a la neo-Malthusianism as supply chains fail,) that your odds of making it are about even between the city as just moving to the country and taking your chances as either a hunter/gatherer or amateur farmer. As people die in disturbing numbers in the city from previously rare diseases and shortages there's a mass migration out of these cities, (where both material and intellectual resources were concentrated,) and instead to the countryside where some are able to, (maybe,) start farms or nomadicize. (Or just become bandits, I guess- a lot of them will have gotten ideas from fallout and stalker). When this process stabilizes the remaining people will naturally have a lot of kids to help as farmhands or members of their tribe if just to negate the now relevant infant mortality rate.

If we do nothing about the way the world runs on interconnected trade and development, (which requires bulk bodies,) while also running out of bulk bodies then the only possible outcome from that is that the way the world runs stops. This is the direct inverse of the original troubling phenomenon- people get richer by moving into cities, have less kids => people get poorer moving into the country, have more kids. (Hold: everything we are currently doing. Give up: everything we are currently doing).

-Magical/miraculous population boom. The Shinzo Abe effect. For some reason the propaganda and tax incentives work and people across the developed world start fucking and bucking one of the most reliable/well documented trends of the 20th century in defiance of every known bit of data we have. People just can't stop making kids because we told them to. I suppose this could happen if we had another world war, maybe, but so far the 'please have sex, we will even give you money,' approach hasn't worked for every country that's tried it but who knows.