Are you sure it’s impossible? and worse yet - can you prove it?
This is known as an argument from ignorance. I’m not sure how familiar you are with this terminology, so to be clear, I am not insulting you or calling you ignorant. But in summary, something is not true until proven otherwise.
The conditions inside the warehouse are not similar to the conditions of the early universe or the primordial soup. You need to demonstrate a mechanism for stable, non-reactive plastic to become sentient if you assert that it’s indeed possible.