I don't think that's a rule so much a consequence of following DRY and YAGNI to the letter. If you don't make something until you need it, and you don't repeat

I don't think that's a rule so much a consequence of following DRY and YAGNI to the letter. If you don't make something until you need it, and you don't repeat yourself, by definition, you'll make generic solutions for when they're needed. The issue isn't the concept and understanding it, the issue is that it's hardly ever straightforward to figure out if you need something, or if refactoring your copy/pasted code so you're not repeating yourself is worth the effort, given there are other things that need to be done. The hard part about software is balancing the tradeoffs between time, money, technical challenges, product challenges, not the ground rules.