"The more you tighten your grip, Tarkin, the more star systems will slip through your fingers." -Princess Leia
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.
But this shows that regulating the market is not a "fire and forget" deal, but an iterative process. First you make the main bad thing more expensive than good behavior. Then you look for "life finding a way", "clever entrepreneurs" abusing loopholes, and make those more expensive too, one by one. You end when you hit diminishing returns.