As someone who uses wired earphones exclusively and must use those USB-C adapters you suggest, it's not quite "just as easy" because there are several problems:
- it's an extra $10 dongle to buy and potentially lose. I've lost several of them over the years
- adds more mechanical stress to the USB-C jack. The office Apple USB-C 3.5mm adapter protrudes out from the phone and I've had several close calls with the wire getting snagged on a door knob which can damage the USB-C port. I've never been comfortable with this Rube-Goldberg dongle contraption that adds more risk to damaging a $1000 phone. It's a fear I never had with the built-in 3.5mm jack on my old iPhone 5. There are 3rd-party right-angle USB-C to 3.5mm on Amazon (including magnetic ones) but the ones I tried interfere with phone cases and they don't sound as good. (Apparently Apple uses a more premium DAC chip in their USB-C adapter.)
- can't simultaneously charge the phone while listening unless you buy a different USB-C adapter that has both 3.5mm input and a USB-C passthrough charging port. These are bulkier.
- it's an extra dongle that's easy to forget. I once got on a transatlantic flight and realized that I forgot my USB-C earphone adapter at home. I panicked and dreaded the idea of nothing to listen to for 8 hours but I was luckily saved by a friend that didn't need to use hers and let me borrow it. Why can't I just leave the USB-C dongle connected to the 3.5mm 100% of the time so there's nothing to forget?!? Because I often need to connect the earphones to things that don't need the adapter.
With all those drawbacks, I still use the USB-C adapters because I have to. But it has definitely made life more complicated.
And if you ever held e.g. Apple's adapter in your hand you'll know how incredibly flimsy its cable is, and how such adapters easily act as levers to mechanically strain the USB-C port. There's a reason headphone jacks are robust - they were actually designed for use with audio devices in mind.
We know as a matter of fact that Android does NOT handle your audio properly when you transmit audio over USB-C then converter. It used to work fine with 3.5mm.
https://android.stackexchange.com/questions/250602/how-to-di...
And you could run into weird issues like this
https://support.google.com/pixelphone/thread/238773737/assis...
Which nobody needed to worry about.
The USB to Jack things are brittle and low quality. That's yet another stuff to not lose and carry around.
Jacks just work. If it ain't broken, don't fix it.
The only other connection type I found to be even worse is microusb. I lost count over how many cables I had to change, some even after just a few months. On the other hand I neved had any problem with usb-c cables or peripherals.
Contrast that to the simplicity with devices that still have a 3.5mm phone - my daily Android phone, my Macbook Air - I can just plug any old headphones in and not have to go searching for the adapter.
And despite the fact that I also own two bluetooth headphones, my wired big-cans headphones have far superior sound quality to either of them. I know it's not a fair comparison because they were well over $100 compared to $10 for the others, but I'm still limited to what I can use them with - which in my case is absolutely everything except my Apple kit (laptop excepted).
In comparison the headphones I've been using have lasted me for over 10+ years with no issue, and any decent high quality set of cans makes the 3.5mm cable easily replaceable.