AIUI, the negative-sum game breaks down like this: consider a black box around the entire bitcoin ecosystem. This black box is consuming $20B/year worth of real resources, which (given that you can't generally buy electricity or generation assets with bitcoin) means $20B/year is flowing out of it.

Now to be sure there are some customers of the bitcoin system, using bitcoin as an intermediary to move money across borders or purchase from certain online marketplaces, and those customers are paying some amount of money into the system to use bitcoin for this purpose (analogous to the 2% that VISA skims off the top when you use your card to buy something on Amazon), but the majority of the money flowing in to balance that $20B outflow is from investors buying in. Unless you think bitcoin has managed to capture something in the order of $20B/year in transaction fees from the likes of Western Union, AMEX and Paypal, what's mostly happening is money being funnelled from investors taken as a whole to the the supplier industries that bitcoin relies on.