Starbucks is big enough they could probably switch over to completely reusable coffee cups and cause the industry to follow. Bring a reusable cup and get your coffee like normal. Forgot yours? You can buy one at or slightly above cost along with your coffee.

Risks to address:

People treat the reusable cups as disposable anyway. I don't see a *massive* chance of this happening given the financial incentive to not do so. But maybe have a discount program where returning used cups also reduces the cost or fills your Starbucks account.

Potential customers choose a different coffee shop to avoid the “cup tax” or hassle of bringing a cup. I *think* Starbucks is large enough w/loyal enough customers that this is also not a huge issue. Legislating it could ensure an equal playing field, but idk that’s the answer.
The second risk could also be addressed with aggressive advertising showing how green it is and how they’re so forward-looking to aim for zero waste. Progressive customers would likely want to support them, and competing coffee shops would also want to piggyback on the good vibes
Is this a terrible idea? I’d love to see them trial something like this. But I think it’s too risky for a smaller coffee shop or chain to try—I feel like you need Starbucks-scale to be the trend setter here.