The biggest problem with making statements about "plastic" is that it's such a broad category of materials that you almost invariably doom yourself to talking nonsense.

Most plastic (particularly PET, PP, HDPE, and LDPE) is not poisonous, even when decomposed, and does not bioaccumulate in the conventional sense. Perhaps by "poison sponge" you don't mean that it's poisonous; you mean that it absorbs poisons. Well, isn't that what you would want to do with poisons? Clay and activated charcoal absorb and adsorb poisons too. That's why you feed them to poisoning victims. The problem is the poisons, not the plastics.

Using alternatives where they exist may or may not be an improvement. Usually you can use glass bottles instead of plastic bottles, for example. But a PET 2-ℓ bottle might be 27 grams of extremely nontoxic PET. The corresponding glass bottle weighs nearly a kilogram, 30 times as much. This means that most kinds of environmental damage associated with it increase by one and a half orders of magnitude; you need 30 trucks instead of one to transport the bottles to the bottling facility, 30 tons of raw material instead of one to make 30,000 bottles, and so on. The glass also needs higher processing temperatures, using more energy, and produces broken glass when discarded, so the disparity is actually somewhat larger.

Similarly, a traditional plastic shopping bag is entirely nontoxic, weighs 100 milligrams, and can be reused three or four times, but is sterile the first time you use it. If you replace it with a canvas bag that weighs 65 grams and can be reused hundreds of times, you're using 650 times more material in exchange for only 100 times more uses, and you dramatically increase your risk of food poisoning from raw vegetables. Washing the canvas bag once will typically use both more energy than the plastic bag used during its entire lifecycle and also more material — it's hard to get it clean with only 100 milligrams of soap!

A better tradeoff is to use slightly thicker plastic bags which can survive dozens of uses, use new plastic bags for your raw vegetables, and bury the plastic safely when you're done with it. (A common pathological effect of regulations against plastic shopping bags is that people have to replace them with plastic garbage bags.)

Being able to use 30 or 100 times less material, and common, nontoxic materials like carbon and hydrogen instead of rare, toxic materials like boron and chromium, are major environmental advantages of many plastics in many uses. There are some uses of plastics which are environmentally harmful, and there are some plastics which do generate toxic products if they are allowed to break down — most notably PVC.

Car fuel consumption is, generally speaking, closely proportional to weight; and, for a given emissions control system, harmful emissions are closely proportional to fuel consumption. Individual car weight has diminished enormously during the last 50 years primarily due to greatly increased use of plastics, though improved metals have also played a role. You can build cars almost entirely from metals, with only a few crucial components such as gaskets and hoses made from plastics, but doing so is environmentally harmful.

So why is there so much anti-plastic sentiment? I think there are four main reasons, aside from the actual environmental damage from some uses of plastics.

1. Plastic is very visible, and renouncing plastic is a low-cost, high-visibility way to advertise your virtue as an Environmentally Conscious Person. Doing things that would actually have a significant benefit to the environment, such as not having children, not eating meat, not financially supporting the US military through taxes, carefully weighing the costs and benefits of possible actions, and using passive solar climate control in your house, is costly and therefore unpopular.

2. I'm a hippie, and the horror at what industrial civilization is doing to our beautiful planet leads many hippies to reject industrial civilization entirely. This kind of anarcho-primitivism, which I do not agree with, considers products of industry and especially petroleum and chemistry to be undesirable, at times even ritually impure, like cannibalism. Thus canvas bags are preferable to plastic bags, wool (such as actual fleece) is preferable to microfiber polyester ("polar fleece"), and brass is preferable to plastic, entirely independent of their actual environmental impact. This creates a sort of coincidental association between environmentalism and the rejection of plastic which provides fertile ground for anti-plastics arguments and barren ground for pro-plastics arguments.

3. At least in the US, the upper class considers wool preferable to microfiber polyester and brass preferable to plastic for an entirely different reason from anarcho-primitivists: they consider innovation and cheap goods to be déclassé, deriving much of their social value from a traditionalist, Romanticist value system. Cynics might also point out that using expensive goods where cheap ones would do serves as a form of conspicuous consumption, reliably signaling the wealth of the consumer to observers. Either way, the upper class — which still controls much of the press in the US, and thus has a powerful role as tastemakers, despite the rise of lower-class celebrities like the Kardashians and Trump — is also fertile ground for anti-plastics arguments and barren ground for pro-plastics arguments.

4. Selling alternatives to disposable plastics is very profitable. As a simple example, a supermarket can sell cloth bags instead of giving away plastic bags for free. Many times, its customers will forget to bring cloth bags with them, and will buy more bags than they need, so in practice a single cloth bag will replace 10 plastic bags instead of 100. This works as a form of price discrimination, since customers on tighter budgets will be more careful to bring bags to avoid the artificially imposed cost. Finally, this makes the supermarket appear upscale, both because it's advertising its hip environmental consciousness, and because it is less associated with déclassé things like plastic bags. This enables it to charge higher profit margins on its other merchandise.

https://en.wikipedia.org/wiki/Microplastics