> But the original work had taken me maybe an hour. And the cost of keeping the data around was approximately zero. It seemed he didn’t consider that.

i guess this is very illuminating - you have to predict the cost of adding YAGNI, before doing it. A low cost YAGNI feature might actually serendipidously become useful.

I feel this is the same principle as random side projects, not done out of necessity but out of curiosity and/or exploration.