If you buy a third party set of headphones that use the lightning port, and require mfi status, yeah, they might make only a small amount compared to profit on iphone.

However, I think enough people will just default to another pair of airpods that any potential losses are offset. I have to imagine that was Apple's thinking. They may sometimes be wrong about stuff, but they're not stupid. If it helps move the market towards wireless, it may also push more wireless beats headphones, which are going to be relatively profitable too.

The quick way to look at this might be: How much was Apple making from headphone sales before this move? How much are they making from headphone sales after?