99.99% of people just want to down a drink and dispose of the cup. Carrying a reusable cup (which takes significant space) is not something most people would do or want.
Too bad, so sad? Environmental problems are something that will require changes in behavior from the general population. I'm not talking hippy-drippy levels of change here where we all start sewing our own clothes, ride bicycles everywhere, and grow all our own vegetables in our background gardens here, I just mean changes where we start using more durable goods and washing dishes comes back as a position at many restaurants.
Carrying a reusable cup (which takes significant space) is not something most people would do or want.
One of the biggest trendy things of 2024 was literally that -- Stanley cups. For 20 years many people have been carrying reusable water bottles everywhere and filling them from spouts at drinking fountains. People who work outdoors (construction, road crews, law enforcement) have been able to get their personal travel mugs and thermoses filled with hot coffee at some businesses for decades as well. All I'm doing is extending that to other beverages. I remember having "Moose" cups from Hardee's in the cupboard at home as a kid. Bring this reusable cup in and get a refill on it for 49! They accepted them at the drive-thru, too. That's how you get people to do these thing, you incentivize it. or you de-incentivize the opposite. I have a couple similar plastic cups from a barbecue chain here now. I remember when we first went (5-7 years ago) the cost of drinks was a little steep, but they came in these cups. If you brought the cup back with you for future visits the drinks were really cheap then. But the cups were the only ones they had. There were no normal dine-in glasses or disposable take out cups to choose instead.
In a modern version of things a restaurant might serve a take-out drink in a cup with a surcharge, or you can get it at the counter in a (cannot take away) vessel with a spout or neck to allow easy pouring into something you have with you.