2000 mercury Grand marquis. Suffered a sudden and horrible coolant leak. The heater core hose pulled out of the "Plastic Ass" circle, and since it was cast into the part, the metal tube, there's no way to glue or adhere the tube back in. (Have tried this at the request of other people, doesn't work but for a few days, given at that point there's three materials that expand and contract at different rates, it doesn't hold.)

Back when, GM had the brilliant idea to use Plastic Timing gears in their 2.2l engines. It did not go well.
Plastic lines, plastic fitting, plastic clutches! It saves weight, money and makes those cars need to line up around the block so that they can be 'serviced'.

This is also a reason why the... past two decades, there will be a 'gap' in the classic car scene.
Mustangs, Fieros, Camaros, whatever gets found in a barn or shed. Grandpa's old car, Grandma's church car. While all of these have some level of plastic in or around them, the major components of the drive train and body/suspension are metal. This means that you can grab some flat beds or a car hauler and take the finds home and begin restoring them, or in the least replace that are rotted or nibbled away to get it running.

2010 Fords with their plastic manifolds? GM's and such? Toyota, Honda, Mazda? Not so much. 20 years down the line there will be very few of these vehicles being 'brought back' into running condition as 'classic' cars of the era. It's already started with the late 90's cars, people snapping up something they drove, their first car model and getting them back into road shape to bring a bit of nostalgia back into the world.

The car that I worked on was already showing signs of serious decay. The fuel injector connectors all had the little holding tags snap under touch, leaving some jury-rigged zipties to hold them in, until the rest of the connector disintegrates... I can get a plastic manifold, today, tomorrow, but the possibility of getting one another 20 years out is slim. And this also won't be the last manifold it gets... cause what with the heat/cold heat/cold heat/cold heat/cold heat/cold heat/cold it goes through, a leak of air or coolant will happen again. Meanwhile you can slap some RTV on a metal one and give it another 20 years with older models of car.

There will be a 'gap' in the cars on the road, 'classics' or just whatever people happen to save and restore will be far and few between. Sure, there may be some models in a museum, but being part of a 'living' and working display on the road brings just alot more to that car than a single place. Cars need to free roam on the roads!

Now, 3d printing may be the answer, once the business casting tools and dies are tossed away. Print yourself something that works, or even metal printers, and some level of tech to properly adhere the layers together. But how many will be left? Probably not many, leaving a gaping hole in the availability of era cars.

Not to mention it puts a timer on cars, gotta go buy a new one cause we used cheap and cost effective parts! You don't want to buy a used one, with the plastic parts that have only a bit of life left! Buy new! Too big to fail! Too dumb to live....

We need another 'Volkswagen' that makes a generic car, with a good engine, put effort in to engineer it to go as long as possible. There's still fucking tractors from the 1920's and 30's that still work. What's your excuse for NOT making a decent car/engine?