As soon as you start mentioning intangibles like charity and reputation and recruiting, a certain group of people will stop listening to you.
I think you can justify open source based on security, quality, and cost. If you open source correctly, you get more secure, better quality, at lower cost.
My personal philosophy is one thing. The stances I employ or postulate to sway others are a sub-set.
On the code quality front, we haven't actually gotten a ton of contributions from the OSS community, but it's a niche framework and not the next Rails, Angular, etc. But what I believed happened when we open sourced it is that we held ourselves to a higher standard of documentations, versioning, testing, etc. Now that we use it across multiple departments with teams that don't really interact with each other directly, having something more stable becomes extremely important. If there is no external accountability I think it's much easier to decide to break an API.