I think you're underestimating most people. Everyone knows they have one life. If they were given a chance to really do whatever they want to do and were asked to think about it, they will always decide to go with the decision that fulfills the top level of maslow's hierarchy which is self actualization. The problem is most people live lives without being aware of this, so I think "Do what you want" is a good advice because it at least attempts to make people stop and think about it.