You go to the supermarket without your reusable bags, suddenly you're _paying_ for the bags.
You better believe that disposable plastic bag usage went _way_ down. People don't like to pay for what they're used to getting for free. It makes them unreasonably angry, in fact.
Angry enough to remember to bring their reusable bags, or to buy some on the spot.
We don't need to know absolutely the exact causal link, we just need to know while in this configuration, this trend is happening. Maybe certain things aren't needed for that trend, but like you say, we don't have the capability of much experiementing, we can have a lot of robust debate and make our best educated guesses and try different things and see what happens.
A tax is another kind of price increase and we do know, empirically, how to study changes in supply and demand. You can get good datasets for it and any company can do experiments by changing prices.
Logic is helpful in understanding how to analyze data but you can't get very far without data.
We don't have time to tinker with taxes in an attempt that we'll prevent total environmental collapse. We need radical solutions yesterday. We shouldn't restrict ourselves to thinking inside the economic system we currently have when it's the economic system we currently have that's gotten us here.