The subtle magic of divergence
Thanks. I also recommend Seth Godin's book 'The Dip' for understanding when to stick and when to quit.
In a nutshell, mastering anything takes hard work (the dip) that most people fail to get through, making mastery scarce and valuable. Instead of quitting things in the dip, work out in advance whether mastery is valuable and whether you have the resources to get through the dip. If so, get started and don't quit. If not, quit before you start and save your resources for a valuable dip that you *can* get through.
Seth Godin also speaks elsewhere about the kind of situation that Harrison Ford was in - it was not enough to master acting to succeed, he also had to attract the attention of and be chosen by gatekeepers. Ford did well to try other things in the industry. Godin says that most of us would do well to avoid ambitions which are reliant on being chosen by gatekeepers.
I strongly suspect that of the actors who have struggled for 7 years, those who persist work acting have a much higher success rate than the one's who quit and become carpenters.
Of course, I'm sure both groups would have tiny probability of success. But the advice here sounds very flawed to me.