Well, those are macroplastics.

I mean, as an analogy think of plastic bottles. Left alone they break into tiny tiny pieces that get everywhere. This is often caused by the sun or abrasion, but underground you've got the preparation materials, rotting flesh gasses, and plenty of microbes.

I would imagine that any artificial thing (including implants) could last forever when left undisturbed for any given length of time

Then you have a fundamental misunderstanding of how things happen, and an oversized confidence in modern caskets.

Not to mention what seems to be a complete lack of understanding what "artificial" means

I mean, cocaine isn't safe for the body either, but try telling that to a big breasted stripper while she's punching you in the face with double D's.

Lots of people make risky decisions...

But in this case it's pretty safe. Ruptures are so common the filling is just as inert as the membrane.

Theyre filled with saline so technically theyre safe. The issue is that most of the time they are not filled properly. Theyre left with a small air gap or have high amounts of oxygen in the saline. This means you can transplant bacteria or mold spores into the saline. Once the implant is sealed, this leads to a contained anerobic environment. This results in an implant absolutely loaded with bacteria where the saline can turn brown and look like apple sauce, or a moldy implant where the saline can turn black or green and become quite firm. If you pop either of those, you die.

Sort of, but it's a stretch on my part. The premise of the movie is that some people have been hired to clean an abandoned psychiatric hospital and, well, some things go wrong.

At some point one of the cleaners finds the crematorium and collects various leftovers from it. That's the extremely tenuous connection I was making.