Great stuff, real pleasure to watch you guys talking.

I have two comments aimed to improve your message even further:

(1) You say:

Money in Steem comes from mining.

Well, technically it is true. But saying this opens you up to accusations like: Well, then it comes out of thin air, it's not sustainable.

I think when people ask:
Where does the money come from?
they actually mean this:
Who is gonna be on the other side of the trade, when you cash out and sell your Steem Dollars?

And I'd suggest the shortest answer to be something like this:

Behind the Steem Dollars that you get for posting are actual shares in the Steem system. So when you cash out you receive "real" cash for your Steem Dollars but at the same time you give up some of the shares in the Steem system. And there are people who would like to buy those shares from you.

Why would they want to do buy those shares?

The shortest answer is that they believe (or speculate on the fact) that Steem (with its Steem Dollar) will one day become a ubiquitous momentary system and there will be millions of people buying and selling stuff for Steem Dollars (which effectively is nothing else than "repackaged" Steem) and, as a result, there will be demand for it as a currency just to sit in people's wallets. It's the same vision Bitcoin (or any other crypto-currency) investors have. It's a speculative bet.

So the answer boils down to this: on the other side of the trade are long term speculators/investors who see in Steem a ubiquitous monetary system of the future.

(2) You say:

This situation where you get thousands of dollars for a post won't last, as time goes by the payouts will decrease, as more content providers join in and there is a limited amount of Steem to be spent on posts.

I have good news: this is not necessarily true. The amount of Steem distributed on posts and curation is not fixed, it's proportional to the market valuation of the Steem system.

Thus, if market valuation keeps pace with increasing competition among content producers, those generous rewards are to stay. (But the opposite is also true - they can drop if market cap drops). Steem is a self-regulating mechanism which spends only as much as it can afford, but it's possible it will be able to afford a lot.