Cable brittleness should be the least of your concern as far as iem construction goes. I've owned iems that broke into two pieces upon being dropped on concrete, and one which fell apart for apparently no reason--until you realize the housing was poorly glued together, rather than using a more secure implementation (I'm looking at you, you muffled-sounding SM3 v1 that costed $300+). One of the most common problems have to do with poor physical connection between the cable and main unit. A brittle cable isn't going to break an electrical connection, unless you go out of your way to mangle the thing. Even then, it's simple to repair with a soldering iron.