The problem is overpopulation is a problem in the areas where there is overpopulation. You can't simplify it to the global average.

People in Japan not reproducing does absolutely nothing to benefit the overpopulation in India, China, and the insane birth-ratios in many African countries. In the same extent, as cold as it sounds, a family of 12 in said countries starving because of too many mouths to feed does not infringe my livelihood.

The harm of overpopulation and underpopulation is largely bound by the regions in what negative impacts they have.

Population growth less than 1:1, meaning that a person will have less two biological children, is a problem for modern countries that is supplemented with immigration.

Our oceans are being depleted, our air is being smogged, and we could resolve so many of these issues if we just allow our population to shrink

We could have a hypothetical genocide of everyone in South America, Africa, and India, halving the global population, and this would have a barely more than marginal impact on the things you just described.

Industry and particular kinds of tech causes those things, not people on their own.

Said improvements to industry to reduce said problems come almost exclusively from the economies of nations that are facing this shrinkage.

North America, Europe, Russia, Japan, Australia, these places do not suffer from overpopulation. Reducing population there further does not help the general problem.

It is only a fear because the people in power want their empire to fill their egos.

It's a fear because of economic viability. The "empires" are not purely a matter of "ego" and it's painfully simplistic to suggest that's the reason.

We need improving industry and economic reforms in modern countries that reduce pollution and make a household with children economically viable for the typical person: instead we're faced with 20 somethings being the new "kids", and people waiting until their 30's to have any children at all.

Places like India and Africa need less careless paternalism from wealthier countries that inadvertently created the problems of overpopulation, and for said regions to have cultural reforms to family expectations. China is still culturally reeling from their past expectations of having many kids, and is suffering from unintended consequences of their solution and how to eventually balance back to a future "2 child" policy.

We don't need more people or less people in the world overall, we need "enough" which varies given certain regions and their respective economic contexts.