Well, two things to consider. Maybe yours are just old and brittle? What I'm thinking is if you did have a metal track instead of plastic, you'd be ripping up curtains before the track. Seems to me they would make the track flimsy on purpose just to prevent that kind of damage from occuring. I.e. the little white thingys sewed into the curtain would pop out of the track before they ripped from the curtain since that plastic thingy's pretty unforgiving. Perhaps if it "did" pop out of a metal track a few times, it'd be worn to a nub and no longer able to hold the curtain up. I'm thinking maybe your plastic might just be a little old and brittle and breaking instead of giving way under pressure. Just a thought...
Happytrails......