There is very likely no direct economic benefit within the next century in the business of getting people from A to B in space … except for space tourism of some extremely wealthy individuals, which may or may not create enough resentment in the long run to be of negative benefit for the economy as a whole.
On the other hand, there may be indirect benefits to be had, if a positive continuing narrative can be created why you are doing it. In fact, there is good reason to create such a story right now: Namely, that a lot of companies have record profits, can’t find investments to generate further profits due to lack of demand (until the private debt load in the USA has been paid off) and thus have little reason to create employment and also very little reason to engage in productive competition. (As opposed to destructive competition by outsourcing, over-capacities, patent wars etc.) All this creates public resentment against corporations across the board.
A concerted effort of corporations to demonstrate that their goals can go beyond mere selfish profits may alleviate this. But here too, there must be both. Tangible results and mile-stones, as well as an achievable and expandable set of goals to keep up public interest. At the same time, the NASA budget is both very small compared to collective profits of major US companies and employed ineffectively through political decision making processes and compensation models (cost plus etc.).
Hence, a relatively small effort can potentially yield astonishing results in this area. (Hardly surprising, given that the Apollo Program is almost half a century old.) This much was demonstrated by SpaceX.
The public has become ever more cynical towards manned space flight, because it has declined in the USA and failed to return anything that wasn’t more-of-the-same for the last 40 years or so. Being first to fly to the moon was achievable, had tangible mile-stones, but you can’t be the first twice. (Which is why Apollo so quickly lost its publicity after Apollo 13.) Having a permanent outpost is achievable, but doesn’t have a lot of tangible results. (Which is why the ISS now only makes headlines anymore when something goes wrong – because continued existence, while success, is rarely newsworthy.)
What it takes is a plan that is a story. One that creates new goals by itself, as the possibility of achieving them arises. This can be achieved on the one hand with a constant improvement in the technology employed or the capital invested. But the former is limited by physics and the latter by economic considerations. (The Egyptians kept putting ever more capital into their big publicity project for 200 years – at which point they couldn’t improve upon the old story and with that, it stopped.)
It is better to use a plan in which past investments don’t vanish. You could try to create organically growing and evolving structures. Under constant investments, such structures will be able grow and provide ever more possibilities, interactions and functionality – not merely sustain an unchanging state of being. If you want publicity, things need to grow and evolve, or they become boring. (Such growth may even create genuine business opportunities in the very long run – but this is beyond 100 years.)
But so far, all structures have been sent to space as-is, to be used and discarded. They cannot be refurbished in space, they cannot be built or created there. That includes the greatest investments – the Moon landers, Mir and ISS. Maintenance of satellites, so far, has been a costly joke.
New concepts must put a focus on keeping heavy structures modular, reuseable and maintainable in place indefinitely. So far as I know, sealing, wiring and plumbing are among the greatest problems there – and solving those would not only help in spaceflight, but especially in all kinds of businesses and products where wiring and plumbing is becoming an ever greater problem in both production and maintenance.
Whether we’re talking about space tugs, space stations or permanent outposts/colonies – keeping stuff usable is essential.
The next step is sending a “factory” to either some asteroid, the Moon or Mars – which can use local resources not only to minimize necessary supplies, but also to necessary investments for further growth. Such that a constant investment can create a growing structure. After that, new goals can be defined without requiring additional investments. And this is a goal that, once achieved, creates such an elusive continuing story, that all the “firsts” of the Cold War, the Space Shuttle and the ISS failed to provide.