Yeah except that happens when people actually stop using something. Do you know how many people use 3.5" headphones? Practically everyone who uses headphones. This isn't some old obsolete technology that no one uses anymore. It's very much vibrant and it doesn't make any sense to drop it now. Personally I listen to audio through a DAC and amp, which will always have whatever jacks nice headphones have (since I use nice headphones), so I really couldn't care less about what the iPhone uses for me, but for other people it seems silly.
It's an older technology that WORKS. Anyone who's into good sound at all will tell you that it's the best way to hear the best sound.
I'm just saying that is not a fair comparison. It's one thing to drop a DVD drive when most people can get music or movies another way. It's not one thing to drop a headphone jack that nearly everyone uses.