[Mondial]

Its not just the high end sets. Audio interstage and driver transformers are a common failure item in general. They are wound with many turns of fine wire and the wire has a tendency to corrode away, especially at the terminations to the leads or terminals. 

A lot has to do with the flux used in soldering the fine magnet wire to the terminations. If it is acidic and the radio is kept in a damp place, the resulting corrosion just eats the very thin wire away.

Sometimes there must be a very bad batch of flux. I have a model 20 chassis otherwise in very nice condition where every transformer and choke has at least one open winding, or has already been replaced. On the other hand, I have another identical chassis where every transformer is original and in perfect condition, so it must be something more than just random failure.