The main reason is not just cost. Modern engineering plastics/composites can be designed to be lighter, very strong, and less affected by temperature changes than older metal alloys. That makes them attractive for lens barrels and even for parts used in top-tier camera bodies.

Compared with metal, composites can reduce weight while still providing good durability, and they may expand/contract less with temperature swings, which is useful for maintaining fit and performance. So while older all-metal lenses may feel tougher because of their heft and finish, modern composite construction can be a deliberate engineering choice rather than simple cheapening.

In short: lens barrels changed from metal to modern plastics/composites mainly for a better balance of strength, weight, and thermal stability, with cost savings being only one possible factor.