I agree up to a point. Certainly the implication you should not continue doing something if a) you think it's fun and/or b) you think it's important seems wrong

I agree up to a point. Certainly the implication you should not continue doing something if a) you think it's fun and/or b) you think it's important seems wrongheaded to me and not a very fun way to go through life. I'm never going to be as smart as Andrew Ng, but I still have fun building ML systems and pipelines. That said, there are times when I've given up and felt like it was the right thing to do. I went to a pretty good school for Physics because I was sure that was what I wanted to do with my life. After 2-3 years, through no lack of effort on my part, it was just super clear I wasn't going to make the cut--I just didn't have it. I worked hard and got okay grades, but as the material got harder, it was just plain to see who was separating from the pack and those kids just had something I never would, and hard work wasn't going to do it. If I didn't put in 10k hours, I came damned close. I dropped Physics as a major, switched to Math, dropped the idea of academia as a career and have never looked back. An honest look at one's talents relative to peers and competitors is still important.