Energy is very easy to produce cleanly (nuclear, and solar is looking better and better). The barriers to doing so are political and financial, not technical. They will be made worse by having an aged-heavy population, which is less innovative, less willing to invest in the future, and less willing to try new things. And it makes a poorer society, obviously.
Malthus has been proved wrong at every juncture. Lack of children is about to bite us very, very hard.
Why is everyone so sure of this? My own instinct is just that we'll adapt to it and it will be fine.
It's just seems like the equivalent of making one of those charts that shows we're going to run out of [some critical resource] by [some date]. They are always wrong, without exception. Because as the resource becomes more scarce, we adapt our societies to become less dependent on it. You can't just extrapolate that trend lines or relationships between statistics like GDP and population continue into the distant future; it almost never works.
I don't have my own competing, grand prediction of how the distant future will pan out. But my thinking is that societies are extremely complex systems and when one variable is tweaked, generally all the other variables also adjust to compensate.
I'm not against growing populations, just not overly concerned about depopulation either.
How? Old people need care. Younger people will need to take care of them. When they die, a lot of knowledge will be lost, because there will not be enough young people to learn all those skills. Less people will also mean overall less economic activity, less creativity, less inventions and so on. I'm not saying the world will stop, but the transition will certainly be painful.
First about behavior in old age. I'm pretty sure the majority of adults today don't want to live and think like their grandparents, so we can't say "less" anything. The circumstances are definitely changing enough that we already know it will be different.
The population decline is not happening in huge step changes. "About to bite us" is definitely hyperbole.
Could it have an effect like the Black Plague, where labor becomes more expensive?
But remember: the uber-rich are allowing millions to immigrate to the rich world, to prevent precisely this. (I'm not commenting here on whether immigration is a Good Thing or a Bad Thing, simply noting that large-scale immigration brings wages down, and that the low birthrate is consistently stated as its main driver.) Also, remember that paying for all those unproductive old people will consume enormous amounts of wealth, something that didn't apply during the Black Death.