USB-C is a much better connector than Mini-B and Micro-B which it is starting to replace. However, I don't think it is as good as lightning, at least not for portable devices like iPhone/iPad. Evidently Apple feels the same too.
You may not have a lot of options for "good" lightning headphones right now, but that will change pretty much overnight if the iPhone drops the 3.5mm output. Companies like Bose that have removable cords will come out with replacement cords with a lightning plug on the end, other companies will release headphones with built in lightning etc. Tons of aftermarket companies will make similar cords, etc. We are already starting to see some lightning headphones, some USB-C headphones etc. and that is without the companies having to do it in order to get sales. They are doing it because they evidently think there is advantages to it.
Just like when the 30 pin was dropped for lightning, adaptors and aftermarket lightning products showed up very quickly. The iPhone is a huge market that companies do not ignore easily.
There was no "reason" to remove USB-A from the Macbook. USB 3 works just fine with a USB-A connector. However the USB-C connector is smaller, simpler, has more features etc so Apple decided to switch to it. Same thing with 3.5mm. There is no "reason" it has to be removed. But if Apple feels like removing it allows them to add more speakers, battery, etc, and lightning is smaller, simpler, and has more features then why not do it?
That's a good point. There are a lot of high quality headphones with removable cables. While they would still need the circuitry built in (and thus cost $$$), I wonder if Bose would make an adapter for my QC15s, like they did when they sold a cable that added a microphone and single button.
Speaking of which, the announcement of the Bose QC35s kept me from upgrading for one reason. You still can't get higher quality Bluetooth from an iPhone. I know android has had support for APT-X for a while now, and some of my Bluetooth headphones support it too. But not my iPhone. It supports AAC over Bluetooth, but I tried it with a headphone that supported AAC and it was a nightmare of pausing and skipping. I hope the removal of a headphone jack brings with it a new standard that can up the quality of Bluetooth streaming with iPhones that Apple approves of. And then it will be the long wait for new headphones to support it.