I think you are not paying enough attention to the fact that we simply don't really know the consequences of plastic in our environment.

We don't know if longer consumption of microplastic has serious health threats, we don't even know the deep sea so we do also not know how plastic affects it.

But what we do currently already know about the effect of plastic in our environment is already shocking enough and with such an ongoing development of plastic we are gonna have more plastic than fish in our sea by the year of 2050:

https://www.theguardian.com/business/2016/jan/19/more-plasti...