With interest rates (much) less than 1% I don't buy your argument. The number of lost sales (I once abandoned my order when they didn't have change - and I didn't trust them to give me a reasonable exchange rate on my Swiss credit card) would well exceed any loss.
I'm an Easyjet CM. We do very occasionally run short of change but more often than not it is £5 notes. Actual coinage is usually fine.
Abandoning an order on the grounds that the crew are short on change has never happened to me and to be brutally honest it seems like you cut your nose off to spite your face. You'd have got your change, the crew weren't going anywhere lets face it. Also, on your card you could surely have gained an idea of the rate you was getting. Plus, with a drinks/snacks order which is typically low value, the exchange rate wouldn't have mattered anyway.
Making a mountain out of a mole hill if you ask me. Yes occasionally crews run short of change but for 99.9% of passengers it doesn't bother them at all provided they get there change by the end of the flight.