Technology

Just leave it plugged into the headphones, don't even take it off. I mean, I have 1/4 inch audio hardware, and I've got 1/8 inch headphones that have a 1/8 inch to 1/4 inch adaptor that just lives on the end.

I totally understand people who want to use wired, TRS headphones. They're inexpensive, widespread, aren't going to become e-waste when their battery dies, aren't going to become obsolete when radio protocols move on, are lightweight, don't suffer from radio interference etc. I have a bunch of TRS headphones and like them. Only downside is that they need some power source if you want to do ANC, but it's not like one has to have ANC.

But...I think that a lot of people are treating it as a "we live in a Bluetooth world or a wired headphones world, and which we do depends on whether there's a TRS jack on the phone itself".

I'd also add that if you have a USB-to-TRS device acting as your DAC, you can swap in others, aren't stuck with the on-phone DAC. I had a phone that had an extremely obnoxious tendency to, when charging in the car, play noise back through the headphones jack (and thus to my car's aux jack and through the speakers). Was fine on Bluetooth. Problem was that the manufacturer had failed to stick the proper filtering circuitry in the power supply for the DAC and was spewing noise from USB power into the audio output, probably because you couldn't see a problem when the phone was running on battery and filtering circuitry for the DAC uses up space in the cramped confines of the phone. (In practice, USB power can be amazingly dirty -- I was astonished watching some people with oscilloscopes look at the power lines on USB.) Anyway, the noise was appalling. If you use the built-in DAC, you can't really change the thing out. With an external DAC, you can stick a reasonable one in.

I don't know how the ones I linked to above perform. But I'm confident that if they are a problem, there are other DACs out there. Whereas with a built-in jack, you get the DAC that the phone manufacturer provides, and clearly some are willing to ship their phones with an inadequate DAC.

I'd kind of like to see someone set up a rig with intentionally-dirty USB power and a bunch of USB audio interfaces and USB-powered devices with an audio output and then see how much noise leaks through into the DAC's output.

EDIT: I also had a (purely analog) audio mixer at one point that used USB power and also leaked audible -- not as bad as my phone in the car -- noise from the USB power source into the audio. Solved that by moving it from my computer's USB output to a dedicated USB charger. I'm sure that there's still leakage and if I were doing pro audio work with that hardware, I'd still be looking at it, but at least it isn't easily-perceptible to me any more.

I think that it might be underappreciated how bad the DAC situation in home electronics is. I haven't seen people trying to measure and quantify it. I have seen lots of people going to great lengths to measure frequency response on headphones, whether or not a cable has (probably completely unnecessary) shielding, and worry about the encoding of their music and sometimes even its encoding for wireless transmission to headphones over Bluetooth. But "how much junk is leaking into the DAC's output" seems to be a curiously un-measured area.