Some thoughts on Open Source:
Level 1- A single person writes a program to scratch their own itch.
Level 2 - That person brings on some co-maintainers.
Level 3 - A company sponsors the software because it is tactically important to other stuff they are doing.
Level 4 - One or more companies form around the software, providing add-on tools, support, and professional services.
Level 1 is the riskiest place for an open source project, Level 4 is the safest place. With each progressive level, you involve more people, decreasing the likelihood that everybody working on the project will say “Meh, this just isn’t my thing any more.” and walk away.
It takes time to make software. Those costs don’t go away when you open source something. In the COTS software model (Commercial, Off-The-Shelf), the price of the software helps compensate the developers’ and/or company’s time. That’s the main incentive to keep building the software.
In open source software, there are still incentives, they just aren’t as obvious, or as easily allocated, as money. But they still need to align with the interests of the people building the software. (And often, the non-monetary incentives are the most powerful motivators.)