Various old phenolics (Bakelite, micarta, etc.) seem to be the only plastics with proven real-world long-term durability. Due to cost and environmental concerns, among other issues, these aren't used much anymore. Like pretty much everything else nowadays, it's unlikely the plastics produced today are as good as these old unsafe/expensive/_______ (fill in the blank with whatever other reasons we hear things are no longer done the way they used to be) materials.
Furthermore, anyone who understands how Business operates knows companies do not build products designed to last a lifetime anymore. Planned obsolescence is the rule for everything from cars and guns to software and fast food menu items. Glock, S&W or any other plastic gunmaker really doesn't care if one of their current models deteriorates after 20, 30 or however many years. I mean, it's nice if they do last (see early Glocks discussed above) but there are no real consequences if they don't. No one is buying those old models anymore, anyway, so the company gets away with it.
Younger guys will try to pretend the plastic fantastics they're raised with are just as good, as legitimate, as steel and wood but count me among those extremely doubtful anyone will be collecting, let alone still shooting, the tupperware produced today 50 years from now. It's all soulless landfill fodder.