"What would you do if you could do anything with your life." Is such bad advice. Of course most people would like to paint or dance or ride motorcycles.
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.
Agree with both of you - it's a valuable thought experiment that everyone should run throughout their lives, and it's also missing the critical check of economic reality.
You can do what you love even if it's not economically valuable... just put your focus on something that is economically valuable and use that as a foothold.
