The problem is we have retirement these days. And also the plague would've killed the old and frail leaving the young and capable behind. Whilst birthrate decline means there are less young people relative to old people. This burdens young people who have to take care of the old. A problem that didn't exist when all of the old people died.

Westerners just REALLY want wealthy, peaceful Asian countries to be uniquely awful.

Those look very similar to me. I would say Japan is now where Poland will be in 10 years. Why it’s a problem for Japan but not Poland?

That's the thing about population pyramids - they don't just move up evenly. They're adjusted by the ongoing mortality of each age group and the size of the next age group down. Poland and Japan are on the same trajectory, but Japan is, effectively, much further along. More ~30-40 years than ~10. The emphasis is less on the largest 'boom' generation, and much more on the general trend of the 'youngest' generations shrinking, growing, or being stable. In Poland, it's uneven - closer to shrinking than stable, but more stable than Japan, which is only shrinking.

Even relatively small differences can have an outsized effect in making the older generations an ever-larger proportion of the population despite their lifetime mortality going up with each age bracket. Compare the percentages here. "Boom" generation aside, Japan's retiree cohort is roughly 150% the youth cohort. That's not a good sign. For Poland to end up with those numbers in a decade, it would have to have effectively no mortality in the elder cohorts - extremely unlikely.

That being said, it is a problem for Poland going forward - as well as many other developed countries.

No one could answer this question.

It is, but not as bad. The EU's birth rate is higher than Japan's, and the EU is much more immigrant-friendly - and if that makes you suck in air through your teeth, let me clarify - that's a relative estimation. SK and Japan are... infamously hostile towards immigration.

Japan and Korea are both further along the path, and moving more quickly. But yes, much of the EU is still on the same path, and the time to effectively mitigate the issue will come and go before most people notice. This is a problem that always has consequences out many decades from current actions, a timeline just too far along for our hunter-gatherer brains to care much about. It will take much longer to be in a real bind, but by the time it happens it is far too late to right the ship.

So humans constantly multiplying and growing has zero impact on the world ?

Only capitalist overlords want constant population growth. Because that's the only way they increase their wealth.

By stealing it from more people. By having more desperate people who know they are easily replaceable, work for pennies.

Exploitation is the game. And you need more people to exploit people. Boys to fight endless wars.

Especially since their influence on the world is likely to start decreasing life span exponentially.

From my reading, eco-fascists demand eugenics, genocide or anti-immigration laws. Pointing out that fewer people on the planet could help, doesn't seem fascist to me.