My phone is relatively large by modern standards and is still thinner than an RCA barrel connector, so it wouldn't even fit. If they were on there, people would probably only connect one cable anyway; I see a lot of teenagers with one earbud in their ear and the other in someone else's, each listening to half a song, or the other just dangling down. :
If anything, connectors will have to get smaller and even more fragile as miniaturization marches on. As it is, the USB charger connection I have now is only a few mm thick, and different than the rest of my mini-USB connectors.
There are a lot of headphones/cables formed for the iPhone, and they all have excessively narrow surrounds, no thicker than the jack itself. I can't see how they are at all reliable.
I've been lucky on that front so far; my mp3 player is truckin' on towards 6-7 years old and all the connectors are fine. It has three 1/8" jacks: headphones, line in and line out. All are still holding solid. My minidisc portable's buttons are all shot and the optics are wonky, but the headphone jack feels as good as new.
Come to think of it, I don't know of any headphone jack failing on any device yet. If anything, the wire will disconnect on the headphone plug itself, not the receptacle on the hardware.