I may not have explicitly stated it in the parent comment, but it's quite explicit other places.
Either a system is sustainable or it's not. If it is, it can continue forever. And, if it's not, it won't. One way or another, it won't, though you can pick the method early on, and later, reality will force it on you. The history of civilization collapse is the history of this reality being forced on groups of people who thought it didn't apply to them.
Plastics in their current form won't exist as new products in 1000 years (though the current stuff probably won't have broken down entirely). Either something far less vile and toxic to "all life" will have been found, or, more likely, industrial civilization will have done the usual "overshoot and collapse" thing, so we won't have the technology to make them in their current form then.
None of that changes the fact that plastic are toxic to life now - and, so, we ought not be using nearly as much of them. I don't mind "durable plastics" quite as much, but the bulk of it is single use, and splitting out all our recycling, I'm regularly reminded of just how much plastic one cannot avoid, even when trying to minimize it.
If the reality (which it probably is...) is that people won't stop doing anything until the external reality we live in forces their hand, the outcomes are almost always far worse than if we decide to stop doing those things earlier.
Plastics are convenient, certainly. They're also a horrid biotoxin that has, quite literally, blanketed the planet in the form of microplastics. We have no idea what to do with the stuff, and burying it only works for so long (and if you're really careful to not let the bits and pieces leech into groundwater). But, I mean, at least you can get water without having to use a drinking fountain!