It's not as though any idea that's at odds with capitalism will require "an alternative system". There's not gonna be any change to the system that requires a new theory of economics.
Suppose bitcoins are at odds with capitalism because they're just like gold (yes I know this is ridiculous, but just for argument's sake). What now? Should bitcoiners make a case for an "alternative to capitalism"? No. You will still be able to buy and sell and own property and trade and own factories and rent an office space and build a company and hire employees.