Ask HN: to open-source or not to open-source?

To open-source or not to? I'm leaning in favor of open-sourcing only the developer-facing code, and not the underlying C library for a few years and then open-sourcing that as well. The project is commercial-licensed. Here's my reasoning:

Advantages of open-source:

+Might get some "free" patches, security audits, and even features

+Might get more sales because developers like open-source

+Easier for the good guys to spot security flaws

Disadvantages:

-Larger competitors with lots of resources can steal the best of the innovative architecture we have and implement it in their products

-Easier for the bad guys to spot security flaws

-Might merge a contribution that infringes the GPL or a patent

That first disadvantage there is the one that gets me, for a fragile startup it seems that a competitor with orders of magnitude more resources could steal our competitive advantage and kill us off before we gain significant traction. Granted they can still do that with reverse-engineering the optimized binaries, it's just hard enough that combined with the usual "not invented here" hubris, we'll probably fly under the radar for long enough. Once we have enough traction it doesn't matter as much, hence why I'd want to open-source it completely down the road. However, I'm open to hearing arguments for and against.