I'm not sure I understand how that argument stands up. Sounds like they're saying "You can plug in better hardware so we're going to give you junk." If they gave us something better as standard, how would that change the ability to plug in something external if the user wanted even better? It sounds like they're kind of showing contempt for their customers in a "You probably couldn't tell the difference anyway so we might as well put more profit in our pockets" way.
A number of years ago, I was given a broken hard drive iPod which I fixed. I never liked the sound of it to the point where I bought a replacement motherboard off eBay because I assumed it must be faulty as I couldn't imagine it being that bad by design. Sadly, the replacement was either identically faulty or they were all that bad. That device's sole purpose was to play music and it had no option for an add-on DAC so what is Apple's excuse for that one?
I can truthfully say I've never heard an Apple device the sound of which I have liked. My Sony MDR-1000x headphones fed over Bluetooth with APT-X kick every one of them I've heard round the room. They don't sound great fresh out of the box on a shop demo but, after about 40 hours of gentle wearing in, they really start to shine. I thoroughly recommend them to you. I'm eagerly waiting for the arrival of Android Oreo which will bring Sony's high-rate lossless LDAC codec to the pairing.