Sites like About.com are essentially for-profit wikipedias (and have often lifted content wholesale from Wikipedia), and they are considerably less popular than Wikipedia, and depend on the whims of Google Search for their revenue.

Wikipedia succeeded as an online encyclopedia by any reasonable measure. That mission is a smaller one than the most ambitious things we can imagine for it, but that's okay. Wikipedia makes a huge impact for its capital cost.

And yes, plenty of for-profits go bust. The fact that wikipedia is considerably more secure is very useful.

Google is great at being Google, and part of that is being for-profit. Wikipedia is great at being Wikipedia, if you accept that it's current state is going to be roughly steady, and part of that involves being a non-profit.