as soon as they melt the system will drain.
They won't melt it there is water in them. So they can't drain. Obviously if they are not filled with water then they will pyrolyse, scorch, char, melt or ignite; depending on what they are made from.
Can't comment on ADB.
I'm not sure that I agree that they wouldn't melt. The 'paper saucepan' works because the paper is thin so the heat is rapidly absorbed by the water. The polymer is likely to be a good insulator & so if subjected to direct flame it'll get above its glass transition temperature (80-odd degrees C if it's unplasticised PVC) on its exposed face. I really can't see a conventional polymer pipe keeping its integrity if
directly exposed to a significant fire, no matter whether it's filled with water or not.
Having said this, I do know that sprinkler systems can utilise polymeric pipework (e.g. CPVC & Polybutylene), but I think that this is normally done either on the basis of the pipework being protected from direct flame impingement, or being protected by the activation of the sprinklers themselves (not really my area of expertise)?