Agree with OP. I think plastic waste is a serious issue but with some effort the impact could be lessened. CX's flight attendants don't seem to have training in environmental conservation at all. I had a strange interaction on a recent flight from HKG to MEL. I was sitting in PE. During meal service I asked the attendant to fill my cold beverage in my tea cup. Her attitude was a mixture of sneering and disbelief - why would I want to use the tea cup? I explained it was to save plastic, since I wasn't planning to have tea or coffee later. She actually shook her head and mumbled something, then handed me my drink in a new plastic cup. Unbelievable.

On other flights, there were times when I was given my cold beverage in two stacked plastic cups. I understand this practice for hot drinks like tea or coffee, when you're given stacked paper cups to make it easier to hold and less likely to leak, but using stacked plastic cups for cold sodas? I assume this is just laziness. Double the waste.