Plastics aren't going to be replaced. In my opinion, plastics don't
need to be replaced. The environmental impact between different kinds of polymers varies enormously. I can't see that there's much truth to a blanket statement such that they're a "major contributor" to pollution.
That isn't to say there aren't problems. Off the top of my head (having taken courses both in ecology and polymer chemistry), you have:
1) Most plastics are made from fossil sources (i.e. oil), and thus contribute to global warming when incinerated.
2)
Some plastics are not bio- or photodegradable.
3) Not all plastics are recyclable, and recycling is often inefficient.
4)
Some plastics contain plasticizers, flame retardants, and other additives which are toxic (e.g. BPA and PBDE).
5) Plastic litter is an eyesore and can damage animal life (e.g. fish stuck in six-pack rings)
These are largely independent concerns, and depend entirely on what you do with the plastic once it's done. If the plastic is burned (in a proper facility), points 2-5 are moot, and you only have point 1. Which in turn is not a high priority because even if every bit of plastic manufactured was incinerated for energy, it would only amount to a few percent of the total amount of fossil energy being produced.
The important thing is that every problem mentioned (save for the last) has a solution, and one which does not involve abolishing plastics. We can make plastics from non-fossil sources. We can make plastics that are more biodegradable. We can replace toxic additives with non-toxic ones.
All this requires is research and the incentive to do so.
People vilify plastic more because of what people feel it represents (= cheap, mass-produced, modern, consumerist, wastefulness, "disposable" things), rather than because of any specific property of polymers. Polymers are an important and indispensable class of materials, and they're not going away. As Mr. McGuire said in
http://www.youtube.com/watch?v=PSxihhBzCjk": "There's a great future in plastics.". And there still is; we just need
better plastics, developed to meet more modern concerns over product safety and environmental friendliness, which do not depend on fossil sources.