Plastic stays with us forever because plastic is non-biodegradable. Biodegradable natural products (wood, wool, cotton, animal waste) can be broken down by bacteria and other living organisms into smaller organic products which do not harm the environment.  

However, plastic is non-biodegradable and cannot be converted into natural waste. 

Plastic will eventually break down and decompose into smaller pieces and release dangerous chemicals. It is said that plastic can take up to 1000 years to decompose in a landfill. Plastic will decompose faster in the ocean than on land but that decomposition in the water will still take a very long time. A plastic bottle can take up to 450 years to decompose.

https://advancedwastesolutions.ca/2020/02/15/canadas-single-use-plastics-law-may-restrict-biodegradable-plastics/

https://refillmybottle.com/this-is-why-you-refill/

https://me.me/i/single-use-swaps-and-the-trash-you-can-save-in-one-21206206

The plastic decomposes into smaller pieces and never disappears. 

Plastic is forever. The EPA has stated that 100% of all plastics ever created by humans are still in existence and continue to pollute our planet. PBS Newshour has reported that since 1950, more than nine billion metric tons of plastic have been produced, an amount reported to be equivalent to the weight of 27,000 Empire State Buildings or to the weight of 1 billion elephants.

http://www.oceansplasticleanup.com/8_Million_12_Tons_Plastic_Marine_Pollution_Per_Year.htm