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Good analysis, and that doesn't even include the phone itself:

Having a 3.5mm Jack on a smartphone:

PRO:

- Compatible with nearly 100 years of audio technology

- Inclusion does not prevent usage of other means of wired/wireless audio transmission - usbc, bluetooth, wifi

CON

- Phone thickness minimum is hard-set to 3.5mm (not even sure this is a con)

- ....cost to.. design? I don't even know.

Not justifying, but waterproofing a phone is very hard if it has holes in it.

Microphone, speakers, charging port - lots of holes. The Sony Walkman Sport from back in the day was waterproof, it had a standard jack

The audio jack is not a hole at all. It is a sealed port with metal plates inside. (Unless you make one for cheapening purposes.)

Perhaps, but Samsung has it figured out.