"Yes, overpopulation will 'adjust' itself eventually, but NOT in the reasonable, placid way you suggest."

Are you saying the so-called "developing countries" (where most population growth is occurring) are incapable of reaching the level of material and cultural development found in, say, Denmark or Japan?

"Even IF we ALL now agreed to reduce populations to sane levels, it would take decades to approach a 'steady state'... and we ALL are not about to do it."

Populations are either dropping or soon will be dropping in developed countries.   They're dropping in many developing countries, too. The birthrate in the middle east is half what it was a few decades ago. Mexico's birthrate is now roughly equivalent to the United States. If you were really concerned (and not merely wallowing in misanthropy) you would want to see more economic development and modernization, not less.

"The people whom I trust have computed ocean productivity, arable land, accessible fresh water, safe living areas...etc., and estimated that the 'optimum population' is somewhere around 2-2 1/2 billion."

Those numbers are meaningless. Thomas Malthus thought he had the game figured out, too.

"We cannot have 153 billion people on the Earth!"

We never will. Point?