The population pyramid becomes inverted with huge numbers of economically inactive retirees dependant on ever fewer numbers of working age people to care for them. Politics becomes hyper conservative as the young liberal voter base shrinks ever smaller. Military strength falls because the number of potential soldiers declines.

Thus, arguably a lower population is the best long term solution for a sustainable human presence on this planet while maintaining or expanding living standards, but indeed this comes with big challenges (which governments and obviously business interests do not want to face).

I don't fully agree with this. Would you rather live on a planet with a population of 5 billion that primarily used coal energy and meat food? or a planet of 10 billion that primarily consisted of renewable energy and vegetarianism?

My concern is more economic and social. With high population levels it seems too much wealth and power goes to the holders of land and natural resources. The Gulf state monarchs and London landlords being the 20th century manifestations of this.

Renewable energy, at least low-zero emission energy, will take over in any case but emissions are only one issue. Our negative impact on the environment goes far beyond that and it is driven by population.