Sorry, I wasn’t very clear. I simply meant the standard has been around forever, not that it was obsolete.

I figure there has to be some reason for it in an age when companies are constantly cutting costs.

6.35 mm jacks are quite a lot more robust than their smaller 3.5 mm brethren.

Which can be a problem when you plug them into the phone - you probably don't want your plug to be more robust than the socket.

you might if your headphones are worth more than your phone ;)

I doubt many people are trying to drive HD-650's with a phone anyway.