The recycling symbol for plastics was a great bit of marketing for the plastics industry. 'Just buy a new thing and no worries you can just recycle it.'
Future geologists are going to see a marine deposit of plastic and be able to date exactly the age of the rock layer.
Yeah, I feel like this Wikipedia graphic puts it quite well why that was rejected:
You see that "Pleistocene" vertical bar? And you see that tiny sliver of "Holocene" at the top. Yeah, the Anthropocene folks were basically arguing that so many riveting things happened in the Holocene already, that we need to declare a new epoch for what's happening now.
Besides, if we do continue to irrevocably fuck Earth and the current mass extinction event continues to wipe out a big chunk of life on Earth, then a future sentient species might declare our entire existence as just the geological event that ended the current era (Cenozoic).