Ahh, I would venture to say that the advice is only as simplistic as you make it. In fact, entire novels could be written about the idea. If you take it absolutely literally, then it is certainly idealistic and optimistic, but I would like to highlight the critical difference with a question: Is your goal simply fun, or it is your true passion?
I did not explain the idea very well, but I would not suggest you pick something just because you did it as a hobby in high school. Instead, I say find your true calling; you'll know you got it right if you can overcome any pains to do it. After all, most people are not REALLY passionate about the roles you listed, they just see that those roles are the ones with the most money and power, and figure they should go in there to live the good life. I am saying that you need to overcome this natural desire to gravitate towards the jobs everyone idealizes, and instead move towards the job truly meant for YOU. As someone else brought up in a reply, there are people that might even love hauling trash. I happen to know such people myself.
Your example of the garage band illustrates this perfectly. The guy might enjoy music, and might even convince himself that this is what he wants, but he goes in there, and suffers constantly. What probably happened, is he saw all the media attention, money, and power heaped upon those successful in the business, and thought to himself, "Hey, I have a band. That could be me." In my view, this illustrates that he did not really understand what the job actually entailed, instead he just saw the rosy pastures, and wandered into the field, oblivious of the thorns. In his place, I would first research what it really means to run a successful band. There are plenty of artist testimonials that explain all the pains that come with the job. If that price is something I can afford to pay in order to pursue this passion, then nothing will stop me. On the other hand, if I feel a seed of doubt that it is worth it; if I think to myself, "Well, it sounds like a huge pain, but the money must be nice," then I will know immediately that this is not the job for me. In that case, I can still keep music as a hobby, so that someday, when I have more time and freedom, I may pursue it at my leisure, but when it comes to making a living, I will find something else to do.
Of course, life is not that easy. As many people pointed out, many roles are already filled in our current system, and the dependence on the huge service industry drives skilled people into dead-end jobs. Still, even in those situations you should constantly explore and expand your horizons. Who is to say your passion will not win out over the tedium of your boring life. It is certainly better than accepting that this is all there is to live. To make matters even more complex, your passions may change over the course of your life. Eventually, you may feel that you accomplished everything you wanted in your profession. Hopefully, when that day comes, you will have a small nest egg to allow you time to train yourself, and reach success in a new field.