Well, my idea is that if 99% of humanity could drop dead by tomorrow, except my friends and family, and those that do useful things for me, that would be a positive thing for me, because the 70 million people remaining would have much more resources at their disposal than the 7 billion idiots running around on this earth.
Life on earth would be a dream again, with 70 million of us.
This is entierely false. Level of life and energy available / head grow exponentially with global population growth, steady. Place with high density of population always have more manufactured ressource at there disposal.
That is first of all only true as long as economic activity is not resource limited ; and secondly, this is mostly also the case because the concentrated areas (the city) are applying extortion on the less dense areas (the countryside), that is, there is a structural flux of wealth from less dense areas to provide dense areas with the stolen wealth ; mainly because the top layers of hierarchy are in the denser areas.
On the other hand, you are right, that structural investments are much more efficient in *sufficiently* dense areas (a road is optimally efficient for a given traffic ; too much traffic jams it, but too low a traffic makes it too expensive per amount of traffic), and costs that scale with distance are also lower in denser areas.
However, I wasn't talking about "putting a human every 20 km" or something like that. You can very well have a few cities with 70 million people. But now we have very, very high environmental costs that orient a huge amount of our production value into being energy efficient, low-polution etc... In fact, we can't sustain that, which is why we have to have most of our industrial production in India and China where the environmental costs are still lower. This is a cost that is entirely gone when we divide humanity by 100. We can drive cars that consume 50 l / 100 km, without a problem, and we have petrol for the coming 500 years or more. We don't have to get nervous about climate change. We can have large forests and still have all the (mechanised) agriculture we'd like.
Our economy is being hurt by resource limits since about 20-30 years and it won't improve. South America and Africa are being urbanized at the cost of huge parts of nature, in a totally non-sustainable way. Until not so long ago, labour was a resource and the more people there were, the more labour offer there was ; it is fading away.
We are now hitting a resource limitation which is strangling us in the same way bitcoin's 1MB block size is strangling it. This has never happened before, because when it happened in western countries, we could colonize, and afterwards, we could delocalize industrial production. We can't any more.
This is normal. Every population is limited by resources if it grows. We could push limits some time. We still can a bit.