While your argument is interesting it is certainly not apt. The largest concentration of plastics is a swirling depression in the Pacific ocean with plastic thick enough to walk on. It kills hundreds of organisms daily, and is constantly growing. However plastics are invaluable in medical science as you mentioned, however the solution to reducing waste of plastic is to eat it. If you break it down into its simpler compounds it is edible, however to me it is still more of a hindrance than a benefit.

The reason why biology works is because energy is flowing freely through and between organisms. Energy flows freely because biological compounds are easily decomposed or can be otherwise changed so that other organisms can use them. But if we take things out of nature and turn them into things that can't be broken down by nature then we have a problem. Plastics stop the flow of energy because they can't be used by nature, which is why they pile up, which places plastics in the same category as mercury and most heavy metals. It's not only reasonable and necessary to say that the use of plastics should be treated with lots of forethought, but also that we ought to change status quo immediately.

There are a two (possibly more) valid solutions to the problem. Reuse everything or use only plastics that are biodegradable. We would reuse all the plastics we are currently dumping, so while plastics are pollutants, they are only a problem due to the way we treat them.