The point isn’t to eliminate literally all plastic use. It’s to save the freaking planet. Taxing plastics based on their harm to the environment has the capability to drastically decrease their use, and possibly (help) save the planet.

Not necessarily. These kinds of black markets form around things with specific demand profiles; plastic for packaging and disposable goods isn't one of them. Also, I'm not talking about putting people in jail for this, just making the whole thing much more expensive (through taxation and maybe extra bureaucracy). People aren't gonna start dealing plastic forks and individually plastic-wrapped cookies on the black market; they simply aren't gonna bother making them and switch to different materials.