>I'm not aware that sesame-allergic people were having allergic reactions to these products in the past

They were. That's the point. Previously there could be sufficient sesame present due to contamination that wasn't listed on an ingredients statement. That's a big ol problem for those with sesame allergies. They're unable to tell from a label whether a product is safe. Now there is a way to tell. That's better.

Contamination is a very real and substantial threat to people with allergies. This isn't hypothetical. People died before we had these regs.

For someone with a serious sesame allergy this change opens up more options they can reliably safely consume. They no longer have to seek out niche producers who controlled for sesame. Now they can reliably read a label and know if there is a sesame risk or not.

This is not a cobra situation at all. Producers putting sesame in products to meet the new standard was an expected outcome, and an acceptable cost. The goal of better communication of allergens is achieved.

Worth noting that these changes take a million years to happen and there's a very long comment period. This isn't something that folks just decided. It's been in the works for like a decade and a half.