"Problem: going to space is so costly, it incites clever engineers to find other far cheaper ways to achieve the same goal. Platinum so badly needed? Well, find another substance on Earth that does it more cheaply!
And thus, space remains prohibitively expensive !
Space is neither the solution to expanding population: there is still plenty of room and the wealthier you get, the less children there are.

The market is not ready for manned space. Far from it! "

Well, I sort of disagree with Dennis. Though I think platinum is a better idea than mining He-3. What Dennis is looking for is a market of stuff in space that can be sold on Earth.

But as far as an early market to sell to earth surface, I prefer, Lunar dirt. Lunar material is more valuable per gram and obviously it is easy to mine.
But any shovel full of lunar dirt or platinum requires cheap lunar rocket fuel, because the cost of bringing fuel from earth to the lunar surface is costly.

So, buying rocket fuel on the Moon is not particularly complicated if you want pay say 15,000 per lb. And if you want say 5 or more tons of whatever rocket fuel you want. You simply offer to pay for it at price of 15,000 per lb and someone would probably deliver from earth. If you want to pay only 2000 per lb you are not going to have anyone deliver it for that price.
If someone was selling water on lunar surface for $500 to $1000 per lb, someone could sell you rocket fuel on the lunar surface for $2000 per lb.

Before someone could sell water on the moon at a low price, it needs to be determined whether there minable water on the Moon.
Minable lunar water depends upon the potential market for water, the amount which can be extracted over a certain time period, and the cost of doing this.
It is my guess, that there is a large need of water at around $500 per lb on the lunar surface and a larger demand for lunar water at about $2000 per lb at lunar orbit.

Suppose you had a choice paying $6000 per lb of rocket fuel at lunar orbit or paying $15000 per lb on the lunar surface.
For this exercise purpose, you can pick one or the other not both- though you could pick neither.
Tick, tock, which would you pick?

There isn't a particularly correct or wrong answer.
Now, lets do one these with a correct or clear answer.
Suppose party A can buy rocket fuel for $2000 per lb on lunar surface and $5000 per lb in lunar orbit.
And party B is not allowed to buy rocket fuel in space.
Which party could bring back to Earth 100 tons of lunar dirt to the earth surface for lowest cost.
Party A should win that contest.

How much would it cost party A to do this? And how much is 100 tons of lunar dirt worth. These are more complicated questions.

Suppose you had a Party C, it couldn't buy rocket fuel at lunar orbit or lunar surface, but it could buy rocket fuel at LEO for 2000 lb. This rocket fuel is brought from earth.
Now Party C can pay someone to deliver this fuel to Lunar orbit and lunar surface.
Who wins, party A or Party C.
Perhaps party C.
And that is why I suggest what I call "water in orbit"- which won't spend the wordage explaining again.

Now, my first post was addressing "Why" not "How".
I think it would perhaps be folly to start today shipping solar panels to space to provide electrical power to Earthlings.
My point is that as far goals for this century and beyond, the purpose of space should be to get trillions of dollars worth of energy from space at a cheaper price than we are currently paying for electrical power.
And the path to this, involves many small steps AND a focus on developing markets in Space for Space.

Having a commercial gas station [or any entity which will sell rocket fuel to any buyer at a certain knowable price with little or no paperwork and waiting in line] in LEO, High earth orbits, the Lunar surface, Mars' orbit, and/or the Mars surface are examples of markets in space for space.