It's a matter of practicality and convenience.
I don't want to have to worry about remembering the phone, the charger, the earbuds AND a dongle. In my opinion it's simply one concern too many for basic functionality. Functionality we used to have.
But yeah, we used to have the convenience of both (bluetooth and wired), now we pay more $ for less functionality and increased inconvenience. Holding on to my SE for the foreseeable.
And you can permanently attach it to your headphones.
I can't permanently attach it to everything I might otherwise plug into my headphone jack, including things that I use occasionally but neither own nor have exclusive control over.
Sure, if I have one set of headphones, and use nothing else with the jack, the “you can permanently attach it” mitigation is, actually, something of a mitigation.
Also, Apple's adapter seems (from reports, including reviews at Apple's site) to either not support inline controls and headset mics consistently and/or to require plugging the adapter into the phone before the headset for some to work. And that's not even going to some cut-price, third-party charge+adaptor device.
I really don't understand people who change entire platforms over such small amounts of money. I know for me personally my investment in apps and content is significantly more than just $2.65.