Apple sold high-quality plastic laptops for a very long time, so evidently there's something there.

High quality for their time. The toilet bowl was very heavy for its screen size, and had minimal volume for battery. The G3 iBook lacked rigidity, and had a tendency to damage the mainboard if picked up from a corner. The G4 iBook had grounding issues, and would occasionally get spicy with two-prong outlets. All three of these issues were directly related to the plastic chassis. All three were great laptops for their day; none would be acceptable in this decade.

There’s nothing wrong with plastic as a material, but there’s a lot wrong with many of the designs of mid-tier laptops that happen to use plastic. The plastic isn’t as much a cause of their problems as it is a signature feature of all hastily assembled corner-cut devices.