Prematurely open sourcing can block innovation
Once you open source your solution, you are subject to public scrutiny. In many cases projects never become popular and it won't matter, but in the few cases that do, it's important to acknowledge that there are risks associated (see: Risks and opportunities of open-source are misjudged.)
Once people start poking around, criticizing, using, and contributing to the project, it will be much harder to perform radical changes. In early stages of development and conceptualization, these changes may be the only way to move forward with new ideas.
With open sourcing, we face the same problem of premature scaling can limit system iteration.
These are the other notes that link to this one.