Almost all our social systems are built on the young providing for the old under the assumtion of generations growing. The population collapse we're currently starting will be the biggest issue in the future. (alongside the loneliness epidemic, but that's a different issue entirel)
We're in for a like a 45% reduction in generation size each generation. And this trend is only increasing rapidly. All the causes of this are deeply entrenched economically and socially, so we won't be able to turn them around on a dime.
Unless we find some social, economic or technological solutions, we are all majorly screwed. Eveyone who won't die within the next 30 years or so will be majorly affected by it.
And no, immigration can't fix it long term, because all the rest of the world is experiencing the same thing. They are just at different stages. India, China, the Americas, Europe are below replacement rate and dropping. All the other regions are are slightly above replacement rates and dropping, except Sub-Saharan Africa. Sub-Saharan African women are having one less child every 10 years, so they will be below replacement rates within a generation. More and more people having internet access will only rapidly increase these trends.
So in 20-30 years, it'll be a zero sum game who can most effectively steal each other's populations.
The only groups that are still growing normally are highly conservative religious groups. Israel is one of the only developed countries that still have normal fertility rates, and they are slowly being taken over by the ultra orthodox.
Maybe life extension or AI can save us, while generally keeping the social order in tact. But all the other solutions don't look very appealing. You could have A Handmaiden's Tale, or government/corpo created babies with artificial wombs, like Bladerunner or Brave New World...
You can't run a society on old people and for those saying it's good because climate change, you won't be able to fix climate change if everyone is in total chaos and only concerend with immediate survival. It'll be like "everything is fucked and YOU want US to stop burning coal, yeah nah".
Well maybe it won't be that bad, but it'll certainly be a huge social issue.