The product for YouTube and Airbnb is the community, not the actual code. Although for YouTube they might want to keep some of the video processing stuff to themselves. As for Airbnb a clone wouldn't take it down because the community is the product. Same can be said of Craigslist.

This is basically the same thing as saying you can't open source until you're firmly in the lead?

No one will see the value in doing what you're doing and want to clone you until then. Even if someone saw the genius in your approach early on and decided to clone you and start competing with you, you have to be confident that your version of your idea will continue to be better than their version. You still have all the advantages of branding, community, and users, whereas their version has no advantages except perhaps a lower price (which doesn't apply to many products, like your AirBnb and Youtube examples).

Obviously I'm biased, but I think this is actually a good method of selecting startup ideas. If open sourcing the code removes most (or all) the value you propose creating, that's a warning sign. Eventually companies thrive by automating tasks with code, but most early stage companies don't provide most of their value in that way. With CodeCombat we intend to either monetize through paid content or recruitment. In either case, having the codebase be open source doesn't impact the value add.

> If open sourcing the code removes most (or all) the value you propose creating, that's a warning sign.

I tend to disagree here: if source code provides your core values, that is a good sign for a healthy business opportunity. Creating your own competition (by open sourcing it) might make sense in some scenario, but it is not a universal good.