Let's face facts.
Plastic is cheaper to tool and manufacture. (lower price to the end consumer)
Plastic is much lighter. (players are tired of hauling 70 lb boards around from gig to gig)
Modern plastics (when properly designed) are pretty much nearly as strong as standard steel or aluminum cases. Carbon Fiber plastics are even stronger!! Plus manufacturers don't want boards to survive 30 or 40 years. They want to sell you the latest and greatest every 10 years or so. Electrolytic caps tend to die on the boards after 15 to 20 years anyway.
Welcome to the cheaper, lighter, disposable world of the 21st century. If I were you, I wouldn't be holding my breath waiting for the metal dinosaurs to return. Aside from a few boutique manufacturers and oddballs like MOOG and Waldorf, you can pretty much expect your Xmases to be all plastic in the future
Gary