First, I was comparing new prices to new prices. Since Canon doesn't seem to be selling the 50/1.8 II in favor of the STM, and since Canon doesn't seem to be selling the 35/2 in favor of the 35/2 IS, I used those.
Second, the Canon 35mm f/2.0 used (via keh) is still $349 minimum. The 50/1.8 II is $80. These are still significantly more expensive than the Yongnuo equivalents. For you and me, spending an extra fifty bucks isn't a big deal. This isn't the case for every budding photographer. Sure, "The difference isn't as high" if you compared used to new (although once you can find used copies of the Yongnuos, I'm guessing those will be even cheaper), but EVEN THEN we're talking a 2x-3x price difference.
Third, and again, my argument isn't "People should buy the Yongnuo instead of the Canon because it's cheaper". My argument is "If you really don't understand why someone would buy a lens that's 1/3-1/5th the cost of the first party lens, you're an idiot".
Your reply is basically "If you ignore the cost advantage, there's no reason to buy the Yongnuo lenses". And, okay, yes, that's right. If you ignore that one little incident during the second act that didn't even happen to her, Mary Todd Lincoln probably had a lovely night out at the theater. To some people, cost matters.