I hear your point that the act or process of doing the learning is good, even if the end result is that you shouldn't do the thing again. Such as learning assembly but then only doing web development for a profession where you don't need to know assembly.
However, I think that the statement below might be better with a bit of nuance.
> It's not true to suggest (or to read these stories as a new bystander and come away with the idea that) "if I were smarter I never would have wasted time on that"
I would say its "not true always," in some cases doing the action really wasn't worth the time.
Related to this, I believe the sentiment people have about regretting wasting time on some endevour, is a misalignment of what their intention was to begin with.
For example, if someone wanted to compile their own kernal because they wanted to learn and understand more about their computer its unlikely that they would walk away from that experience with regret. However if they wanted to compile their own kernal because they believed that in doing so they would make 10x more money in the long run (through learning so much), and that goal failed to materialize. They would likely tell others to not waste their time learning to compile their own kernal.
Not trying to be pedantic, or argumentative, I aggree with your point deeply, however I wanted to discuss it a bit further. Let me know your thoughts.