I am all for minimizing/eliminating single use plastics.
But when i get served a milkshake in a plastic mug, with a plastic lid, and a plastic spoon, but a paper straw because of "save the sea"...
What if dispenser machines had a pay by volume model? You bring your own thing, they fill it, and charge you by how much you use. Would probably need something added to measure flow and set prices, but it's not like a McDonalds built in the 70s is still using exactly the same machines they were back then.
Could just do it by weight. Put vessel under nozzle. Zero scale, and hold till weight determined for sale, hand to customer. Could likely even have software do it.
Years ago at Universal for Halloween Horror Nights they used bottom fill beer dispensers. They had a connector on the bottom of the cup so you could grab a cup pop it down on the machine and keep going. Say 15 beers in seconds. The beer fills to the exact height needed with the exact desired foam amount on top. No over poors or needing to have any loss. Time was cut down drastically. The cups had to be expensive, but when your charging $10 for $1 with of product you don't have to worry to much about cup cost I guess. I remember thinking at that moment American Capitalism has peaked haha.
Honestly how much more expensive would glass mugs/cups be? Like A&W Canada will give you a chilled mug for root beer (and other drinks but the root beer is iconic)
If it's to go then then paper cups are fine. The paper straws are just annoying...