Don’t have an iPhone, don’t want one (though I do have a couple of old iPods somewhere, and an iPad). But, I think the issue here is at least partly the lack of anything better being substituted. Something better may come down the pike, but right now, there isn’t anything stepping into the void left by the 3.5mm jack. It doesn’t appear as progress when you take away something on the promise that, eventually, something better will come along to replace it. It seems to be progress if you take whatever it is away, and at the same time, offer an alternative that can be considered better.
In this case, yeah, analog connection technology is eventually going away; the TV I just bought has no analog outputs at all. But for phones–and a lot of people use them as their sole music device as well as their phone–an analog jack has been something people relied on. They have headphones–sometimes expensive ones–that use the technology. It’s possible an adapter will sound the same, but it’s also possible the adapter will muck things up. There are costs to using the same port for different things, such as lack of simultaneous charging and listening. In return, you get the arguably progressive step of moving towards a new paradigm for all connectivity, where wireless isn’t a supplement but the default, but you don’t get any real progress towards that goal. Just the negative step of eliminating the old paradigm.
It might be necessary in the abstract, but by not really offering anything to replace it Apple comes across in a less than appealing fashion to a lot of people apparently.
For me, I’ve never listened to any music on any phone I’ve had, and the only thing I’d connect to my phone would be a Bluetooth headset probably. And I already got my hissy fit about Lightning connectors obsoleting my iPod accessories a long time ago.