Sorry to hear of your malfunction.
My experience is that plastic isn't always cheap OR fragile. Plastic/nylon compounds work extremely well under design conditions. As pointed out, outside design conditions they can become brittle, etc.
Would metal be a WORSE material for a tensioner rail? My thinking is that it would introduce small metal particles in the oil during the life of the rail, and being as hard as the chain, would shorten its life by wearing it down.
The above wondering comes from observing a plastic/nylon, not metal, guide block on the swingarm of my motorcycle where the chain can rub at certain angles. This block will outlast several chains...
Maybe I've got too much free time on my hands, and had to put in this $0.02. Nah, I'm just addicted to this place.