But there are lots of crowdfunding successes for videogames, even though they present the same problem that you don't know in advance how good it will be. So I don't think the situation is hopeless.
Another idea which I think has promise is payment by social conventions. Tipping culture in the US is like this: you are not legally required to pay waitstaff, but everyone does because they would be considered assholes if they didn't. People joke that it's ridiculous that rock bands and web cartoonists make most of their money selling t-shirts, but actually if we are aiming for social pressure t-shirts are the ideal currency---everyone you interact with in your daily life can see that you bought one. So if we got a convention going that "if you regularly read a webcomic you oughta buy the t-shirt", then it would be easy to ostracize the people who violate it.
Another idea which I think has promise is payment by social conventions. Tipping culture in the US is like this: you are not legally required to pay waitstaff, but everyone does because they would be considered assholes if they didn't. People joke that it's ridiculous that rock bands and web cartoonists make most of their money selling t-shirts, but actually if we are aiming for social pressure t-shirts are the ideal currency---everyone you interact with in your daily life can see that you bought one. So if we got a convention going that "if you regularly read a webcomic you oughta buy the t-shirt", then it would be easy to ostracize the people who violate it.