The ultimate incentive humans can get from open source is paving the way for utopia.
Of course, there’s also saving governments tons of money, software freedom and sovereignty, etc.
However, the thing that actually blocks this, IMHO, is that humans (in general) want to always be competing and feeling better than each other. In a utopian world, the only competition would be sports, games, etc. — not “I have a bigger house”, “I have more money”, etc.
Individualism, the by-product of capitalism, has made us only care about bettering I.