Firstly, a marketplace is just one example of profitable ventures Steem can leverage. There is also paid advertising, gaming, gambling, mutual aid societies, web hosting, decentralized exchange (Cryptos and stock/commodity derivatives, etc), among others.
Dan has failed miserably at DEX so far to understand that people don't like change (most stick with the centralized exchanges). Everything he has done has failed to gain adoption. So now he got desperate and decided to give away money, which of course is popular. But he still hasn't proven he can create an income stream business model. Afaics, he has only shown he can deceive (with the improbable high payouts dangled in front of their faces) and bribe bloggers.
Secondly, you can't be sure that 99% of things people will want to buy will not be sold on the marketplace. Merchants hooking up with drop shippers could cover much more than 1%.
Try to get out and think about relative size. There are 6 billion people. Do you think you can put even what 600 million of them want and how they want it in your marketplace. How can you put paying their rent and buying gas in your market place.
Even you only have a million users, your diversity of wants will be the cross-section of the 6 billion.
Thirdly, it will be much more convenient for the Steem market place to purchase things with SD in a Steem marketplace, and they would probably pay a small premium to do so. Exchanging SD to STEEM to Bitcoin can be inconvenient, and the fees involved may make up for the premium on the cost of the products or services.
I think it will be more hassle for them to try a new marketplace and new suppliers for their products. People are creatures of habit. It is very difficult to change people. Have you not had a gf yet and learned this?
If you make it a huge hassle to cash out to fiat and BTC, users may lose interest in trying to earn.
Fourthly, confidence is built overtime. Amazon and Ebay did not start off with their loyal user base from the get go. It is not a stretch to say confidence can be obtained over time.
Perhaps over time, but for example I'll be coming up behind you soon enough and offering something much better. So will others, including Bitcoin and ecosystem marketplaces built on it. The world won't standstill to give you endless time.