No one uses cash at starbucks. It's all phones, credit cards and debit cards. Try and buy a beer on a plane. They don't take cash.
I don't know where you live, but in the UK cash is still common, even with tap and go on cards. Also what planes have you been getting on? They usually take cash in a ton of different currencies.
Every plane I have been on in the US refuses cash and only takes card.