and if I don't get a plastic bag... the store still has plastic bags regardless

that's my point, that framing has cause and effect backwards! It's how the capitalists want you to view it; as though consumption causes things to happen. They want us to think our leverage comes from what we choose to buy. It's not. They've already made the decision, already built the infrastructure and set production in motion. At best, we decide the rate it gets consumed. Maybe, maybe if you tie it a strike - the thing that actually gives us leverage over the system, and the thing they desperately would rather us not talk about - a boycott can achieve something. But on its own? Abstaining can't eliminate something from being made, it just makes space for one more option.