Our current economic system focuses on money as a medium of "equivalent exchange", as in there's no free lunch and a dollar you spend on buying a piece of butter necessarily means you can't spend that same dollar on a gun (same thing for the butter merchant). This exchange is based on a contractual agreement that binds for a finite time, and stuff that happens outside that window is necessarily considered an "externality". In other words, money facilitates a social interaction that prioritizes focus on the costs/benefits of individual(s) involved while de-prioritizing the costs/benefits of the collective at large.
But contributing to open-source is not that sort of social interaction, instead, it's one that prioritizes contributing to the collective over individual gain - i.e. a social interaction that focuses on externalities instead of on contractual time-based exchange. Money, with its focus on being loosely "conserved" (e.g. you can't double-spend the same dollar), is simply not a good medium for encouraging cooperation and discouraging free-riding to open-source (and other public-goods)
In traditional villages, public-goods contributions were facilitated with some combination of reputation and gossip. Later, cities and states managed to come up with taxation, courts, and prison when reputation and gossip failed to scale. But the super-connected capitalist "globopolis" of today demands a new form of organizing cooperation and eliminating free-riding when it comes to public goods.
To properly solve the problem of the economics of open-source, we need to solve this more general organization problem.