It's not free to design or sell, even if the audio parts used "have the capability". If Apple thought they'd sell more phones, they'd do it.
Back when I was fighting the good fight against Apple, MP3 player vendors kept adding FM (and eventually HD Radio) in the hope that it was what Apple users were missing... The radio industry liked that strategy too.
Ugh, this has always been a terrible idea - apparently a vanity project by people who know nothing about how manufacturing works.
Every piece they make modular adds cost - cost of connectors, weight of the frame to support them, cost of support when the connectors fail.
You can go on about "waste" all you want, but the lowest cost, lowest material devices are rated for single-insertion connections along with components both stacked and heavily integrated.
The reason they could not do displays, RAM, etc is that these components are tightly integrated to the SOC selected, and change regularly. This was true when they announced, even more true now.
Truly Google is the new Microsoft if they devote such time to obvious executive vanity projects, untouched by the taint of reality.
There's worse problems at a lower level - hardware caches on the disk do not really guarantee flushes are honored either. There's a discussion of this here - honoring writes is an enterprise feature... http://serverfault.com/questions/460864/safety-of-write-cach...