Already for a long time there is a system of payment on the fact, that is advertising is paid for each given client. this has been present for a long time and quite successfully.
The final meaning it does not change. There is an amount of work (advertising) there is a conversion (customers) if the ratio does not satisfy the demands of the advertising company then it simply will not do this work.
In the example that you bring the conversion depends on the product. So it must be excellent.
Yes, a lot depends on the product. At some point, it begins to advertise itself, but before this moment advertising is necessary.
Often, projects that go to ICO launch advertising their product, without having a working prototype and even a less competently designed smart contract. To make the advertisement really effective, the project should have a ready-made prototype, otherwise it's money wasted.