This argument fails to follow on two fronts: 1. That decision seems unlikely. "I want $X. But I'm worried that if someone copies my idea I'll get $0.2X. So

This argument fails to follow on two fronts: 1. That decision seems unlikely. "I want $X. But I'm worried that if someone copies my idea I'll get $0.2X. So I'd much rather keep it to myself and get $0 instead." I'm no fan of the assumption that humans always make rational economic decisions, but surely it isn't normal to be that silly. 2. The assumption that if the first inventor doesn't share their idea then that idea is lost to humanity and nobody will ever come up with a similar idea ever again just doesn't follow. We've got major business strategies (submarine patenting) and even an entire industry (patent trolls) who rely on the fact that this isn't how things tend to work out.