I think it's more a question of what someone wants out of life. If your job is your life, than that makes more sense. If you value things outside of whatever you do for money, then working to burnout is only going to harm those other aspects of your life and ultimately you.
I don't think it's unreasonable to put some grey space between being the best you can be and being laid back. Something like working reasonable hours, being thoroughly successful in your career, and still engaging in outside activities. I think that burning out=being the best you can be is pretty shortsighted.
Also, for the kind of employee this is about, "your job is your life" is unlikely to be true, at least not in the meaning that you imply. "Your jobs are your life", in some cases is appropriate, not because they enjoy their job, but because there's so little else.
I don't think it's unreasonable to put some grey space between being the best you can be and being laid back. Something like working reasonable hours, being thoroughly successful in your career, and still engaging in outside activities. I think that burning out=being the best you can be is pretty shortsighted.