Yes I realise all of this. To clarify, I would deal with the swapping of a 3.5mm jack in the iPhone if it swaps to a new standard, whether that be USB-C, micro-USB or whatever. I reckon if it swaps to a standard that everyone can get on board with then it won't be an issue for long. USB-C is a good example because every new device is likely going to be shipping with it. One wouldn't need the adapter for long. Replace USB-C with 'some new standard' if you prefer.
I'm not sitting here saying we need to keep the 3.5mm forever, I'm just saying a swap to a new standard is better than not, especially for something so common. The deal breaker is if it swaps to lightning, which will likely never become a standard on other devices, so you will always need an adapter.
With all due respect, if Apple switched to USB-C, and gave you a free set of USB-C earbuds, even if Samsung and Android hardware also switches to USB-C that's at least a year away if nor longer, before only NEW devices will have it, which at best only gives you compatibility if you switch between new iPhones and Android phones. It also only applies only to brand new PCs and Macs. I don't know about you, but I'm currently using an iPhone 5S, an iPad Air, and a 2014 MacBook Pro. Other than the phone, I'm not planning on upgrading any of them any time soon. And that will be true for the vast majority of current hardware in the world. From iPods, to PCs, to stereo equipment, it's going to take over a decade at least before all of that old equipment is replaced with USB-C, and that's from the point where the entire audio industry embraces USB-C as a standard. You'll be carrying an adapter for several years to ensure you'll be able to plug into any device you happen to come across during your day.
Now consider if you will, USB-A has been the standard since 1998. That's 18 years, which is a good healthy run, during which there was USB-B, USB 3.0, FireWire, and Thinderbolt, competing for the standard. So what makes you think that even if USB-C becomes the standard across the board starting today, that USB-C will even last 18 years with all the new technologies swirling around out there? And even if it does, more than half of that time you'll need an adapter to connect to older equipment. So you might get a few adapter free years after that before the next standard comes along. And I think that's being optimistic. USB-C is a great connector compared to USB-A, but in a lot of ways it feels like an evolutionary step in the right direction, but not a standard. And the last thing anybody wants is to invest into a lot of specialized equipment that will eventually require and adapter when the industry moves to the new standard, just like all of my USB-A equipment (which I'm not going to just replace) does now.
If Apple does this, and the rest of the industry follows only using USB-C, it's going to take years before you'll be able to use native USB-C equipped headphones without an adapter on every piece of equipment you may need to connect to during your day. So it kind of doesn't matter whether it's Lightning or USB-C. You're going to need an adapter, and there will be Lightning to USB-C adapters as well. So it really all depends on what your needs are. Some Apple people likely plug in their headphones only into Apple products. Some will only need Apple and 3.5mm products. Some like you will need Apple, 3.5mm, and USB-C. In which case, since you're going to have to carry an adapter no matter what, you're better off keeping your 3.5mm headphones and getting an adapter like this:
But rallying around USB-C as a standard is not going to change your need to carry around at least one adapter for many years. With that in mind, for me it doesn't matter if it's Lightning, USB-C or one like the above. But since I don't have as much confidence as you in USB-C becoming the standard for another two decades, much less longer, I'd just as soon not invest in a bunch of new specialized equipment that may be supplanted in a few years, since I'm going to have to carry around an adapter anyway.