I'll begin by clarifying that breast implants are not plastic, they are made of silicone. Silicone will degrade over time, maybe a long time but it will degrade nonetheless.
Breast implants are designed to be strong, but they still can break. That's why the medical profession has switched (for the most part) from a silicone gel to a saline solution. The shell of the implant on a saline filled implant is still silicone but won't cause the damage the gel would. The saline solution is just salt water, harmless, although I'm sure it's still not very pleasant to have one of these break inside your body.
There is no expiration date for a breast implant, the body will break it down over time, but while one woman may only have an implant five years before it breaks, another may go fifty years without it breaking.
The stages of decomposition would be much harder on an implant that a living body would. For one the body will become dry and sharp in certain places, specifically the ribs. The chances of some part of the body puncturing the implant is much higher, add to that the regular decomposition of the implant and any little creatures that may have moved in with the body and we see that the implant cannot last for very long.
For arguments sake let us say the implant lasts longer than the body can in the grave. The answer is still no, the breasts will not remain intact, as the skin and muscle degrades so does the substance that holds the implant in the body degrades. Eventually the implants will literally fall out of the corpse. This fall may be what actually breaks the implant.
For more on breast implants check this site out.
http://www.implantinfo.com/
