@GauthierThere are some interesting psychological tricks that can make Pay-what-you-want pricing work in limited situations, but that doesn't change the problem with the PWYW model.
For example, if you give someone something absolutely free then offer another thing at PWYW, you are likely to make more overall. So free coffee plus PWYW muffins makes more money than PWYW coffee and muffins.
Also, people are more likely to give money if it's going to a cause than for profit. So PWYW coffee will make less than PWYW coffee with all proceeds going to the SPCA.