About the plight of Elephants and efforts to conserve them?
Dehorning of rhinos has been tried with limited success. Poachers have killed dehorned rhinos anyway, either out of spite or to avoid tracking worthless prey in the future. There is the problem of anesthesia (always a risk) and the fact that rhino horns are usually not destroyed but saved in the event of decriminalization. But at least horn grows back.

Considering the strength of horn and ivory, and the force applied in their use, plastic would not be of much use to them (especially elephants with long tusks).


  They are used for digging for water, salt, and roots; debarking or marking trees; and for moving trees and branches when clearing a path. When fighting, they are used to attack and defend, and to protect the trunk. - Wikipedia


To detusk elephants would be akin to declawing a lion, though much more dangerous and painful. Though this might help prevent poaching, the future of these animals lies in prevention.