The purely logical standpoint would be to head out tomorrow and 'remove' 75% of the human population. I reckon about 2 billion is a sensible and sustainable world population.
Obviously morals and ethics get in the way of logic from time to time. For anyone with a brain, the obvious thing to do about an ageing population and/or a declining population is to figure out how to work with the demographic change. The incredibly dangerous and naive thing (hence it's what governments of all colours have done for decades) to do is to try and inflate the population to deal with a perceived shortfall of workers.
Spoiler alert, immigration on average is a net economic drain on society in the short and medium term and only reaches equilibrium economically after decades. Bringing in workers to deal with a shortfall is treating symptoms not cause and simply pushes the problem down the road to be an even bigger issue down the line.
Why does nothing work in the UK - let's have a think - oh yes, population has increased by around 14%, >8 million people, in 20 years - I wonder if that might not be a considerable influencing factor? The previous 14% growth took around 45 years (which includes the post-war rush for workers...)