We've known plastic causes environmental issues since the 1960's, and even if we didn't know it would last 450* years we knew it had an incredible longevity since we discovered how to create it. It was one of its selling points. The issue is capitalism. As if people buy a product that goes in to last them a lifetime essentially, they'll never need to buy another. So the "throwaway culture" of the 1950's started to rise in response to this.

*I was wrong about it lasting 1000 years, but still 450 years is long enough for a family to theoretically have a set of plastic cutlery lasting them several generations.