Environmentalists welcome lower world populations. In reality, the global population is due to increase for at least another 25 years, thanks to Africa. The continent is also industrialising rapidly, cancelling out what's happening in Europe.
Older and reduced populations in the first world mean a more inefficient use of resources, such as mass transit designed for large populations. Innovation is often driven by young minds, decreasing the odds of solving problems such as the energy crisis and environmental issues.
Governments will be forced to raise taxes (+10%), raise retirement ages (to at least 70), import immigrants (3 million in the UK) or likely all 3. These will be deeply unpopular with the people.
Immigrants who are of no benefit will trigger the rise of right-wing populism. Rich people and businesses will relocate to foreign tax havens.
The cost of living will explode, incentivising people to have fewer children, creating a vicious circle that we are already seeing in China. Young people are already giving up on careers or owning property.
The pension and healthcare systems will be crippled, and many people will be forced to care for ageing parents themselves. Those with children will have to balance the two.
Political parties will court the "grey vote" since older people vote more. Young people's issues will be ignored in favour of pensions and healthcare. Alienating them even further.
My dad's own life has improved dramatically (he was a virtual recluse) since my sister gave birth to twins. If half of 30-year-old women today are childless, that's an epidemic of elderly loneliness in the future.
Old people are starting to form parallel societies; some coastal towns already have an average age of over 60. Young people will move out, leading to a significant decline in intergenerational mixing.
Old vs young will be a future battleground. Young people who can't get on the property ladder will deeply resent the old for hoarding all the wealth.
It's going to get messy......