I often applaud Apple's decisions to jettison legacy technology for new standards, precisely because that drives new technology. This is different.
First, headphones are audio products more than they are technology products. If you have a great pair of headphones, you don't necessarily "upgrade" to anything, ever. Rather, your goal is to enjoy those headphones as long as you can.
Second, great headphones are very expensive. Those of us who have invested hundreds or thousands in headphones are motivated to keep them for that reason alone.
Third, while adapters exist and Apple surely will make one, having to use an adapter makes the experience worse than it would be without the need for one. If for any reason you can't or won't replace your headphones, that makes a new iPhone inferior to the iPhone you already have (in that respect). While adapters aren't ever fun, they usually are more of a temporary measure than they will be for many in this case.
Fourth, the inconvenience of having to use an adapter is intensified by the fact that iPhone is a portable device. Having to carry an adapter all the time seems unworkable, yet that is exactly what many will need to do.
Fifth, the intended replacement technology -- wireless -- isn't as good as the wired technology it replaces. If you're an audio enthusiast, you probably will notice the difference with wireless.
Sixth, the emerging new technology isn't widely adopted, something that makes a big difference when jettisoning the old one.
For those reasons, Apple would be better off keeping this particular "legacy" port for at least a bit longer.