So, what would motivate states to go down the long, expensive and slightly crazy path to build space habitats? There would have to be public pressure to do so.
The first case for it is moral, and related to the environment. While we may be able to reduce the world’s population in the very long term by emigration to space, and thus ease pressure on the global ecosystem, it won’t happen for a long time. We can, however, actually expand the habitable ecosphere beyond this planet for the first time in the Earth’s history, partial compensation for concreting and paving so much of it over. It would be a first step on a process which would have almost no practicable limit.
The second reason is that it provides an exciting objective. In the developed world, we have on average long, healthy prosperous and safe lives which our forebears could only dream of. Yet we get used to it all too quickly : “Is this it?” (China, that is your future too, in a generation or so). Boredom is the main enemy of peace and prosperity. Here is an outlet for heroism which does not involve adhering to a crazy murderous ideology or religious cult.
A realisable objective is to campaign for 1% of GDP in developed countries to be spent on space development. It’s a lot less than is spent on defence, not to mention more frivolous consumer goods…
A critical path for this objective must take into account that economics dictate that space habitats must be constructed in space, even small ones. That means a lot of investment in infrastructure and space manufacturing before any habitats can be built:
A manned landing on Mars,currently a target, seems a distraction from this.
Only then can the first habitat be built.
National Prestige – and Military Strategy