The real root of the problem is that there is no root of the problem. There are hundreds of causes that all feed back into one another. If you try to boil this down into a root-cause narrative, what you will inevitably end up with is a cartoon caricature that, at best, pushes an anti-human narrative; and worse / more likely, blames some proxy for the poor, racial minorities, or foreigners. I know this because the whole 'population bomb' thing wound up being used by fascists to try and paint their shenanigans as environmentally friendly.
[0] In Russia, this is literally the only thing that will get Putin chucked out a window. A very large chunk of the country moved to mining towns in the 70s and 80s to take advantage of the Soviet Union equivalent of FIRE[1]. Those pensions are a crushing burden upon the government and any attempt to reduce them causes massive protests and riots.
[1] Financial Independence, Retire Early
Sources? In particular, what proportion of the population?
When were the attempts to reduce the pensions?
This video has it's own sources list linked in the description: https://pastebin.com/AbaJ8EW1
To summarize: in Soviet Russia you got early retirement if you worked in mining, or moved to certain smaller towns. A lot of people in Russia maximized their early retirement by doing this. Then the Soviet Union collapsed and Russia went through Hard Times. Their fertility rate fell off a cliff and never recovered. While other countries also had falling fertility rates, they also have lots of immigration to offset that, but nobody wants to move to Russia. So imagine America, but with no millenials, no immigrants, twice as many boomers, and they all only care about keeping Medicare functional for exclusively themselves.
One of Putin's first acts was to attempt pension reform, because it's the $3600 spent on candles[0] in the Russian budget. This led to massive protests, so he backed down. He tried again around the time COVID started, and again, massive protests. It's the one thing that reliably causes Putin's popularity numbers to fall. Not even an unpopular invasion will do that. Because, for a large proportion of older Russians, the government exists solely as the medium by which treasure is pilfered from the rest of the world and into their pocket.
I don't remember the exact proportion of Russian pensioners to the rest of the population, I just remember that it's unusually dire.
Your point is a red herring.
[ The means are my calculations on dataset in https://www.pop.org/simple/countries-with-below-replacement-... ]
There are some negative consequences or at least difficult questions to wrestle with. Where is the tax coming from in a massively aged world? Who pays for healthcare for the elderly and who looks after them? Will retirement remain an option in those countries and will those folk get a pension? Others can comment on the positive aspects of declining fertility.
But I don't blame most individuals: people buy what's available in shops and have little choice to do otherwise. The problem is in the economical system.
The population is going down. People aren't having enough babies for replacement. You're going to have a rude awakening when you get old and there's not enough people to take care of you, pay into the retirement system etc.