Dropping the DVD drive was also a pretty big thing at the time as there wasn't even a 'similar' alternative (another type of removable media the industry had switched to. In the past it seems when they drop technology it real does make the changeover period quicker. This time I'm not so sure it will but at the end of the day I don't think it matters. Most people use the headphones that come with their iPhone and they will continue to. Not to mention Beats has 60-80% of the headphone market share depending on who you believe so with Beats + EarPods + AirPods most people are covered.
"Most" people do a lot of things, but that doesn't mean that use of proprietary patent protected communication standards doesn't really hurt innovation and customers in the long term.
Was that downward slope clear at the time Apple made the changes, though? I remember "no Flash on iPhone" being a major complaint of many pundits.