I would disagree with your thesis that 99% of the experience is based on pursuing your idea. Many entrepreneurs start companies for reasons other than pursuing their own idea. Some simply like the control, autonomy and culture of a startup.
I'd also posit that when finding co-founders, you are in a sense finding someone that will help you execute your idea (and that makes them no less of an entrepreneur).
"Some simply like the control, autonomy and culture of a startup."
Will people really get the opportunity for that, though? If the idea isn't "theirs" in the sense that it was (more or less) handed to them as something to work on for you, will they feel the same freedom to change or modify it as they would with their own startup? And isn't that flexibility a necessary component for a startup's success?
It's a delicate psychological issue... as students, we've been very much conditioned to respond to an authority figure (teacher, boss), and if you don't want to fall into that role, you should be consciously careful not to.
"Some simply like the control, autonomy and culture of a startup."
But those are the people that work for startups, as early employees. And they can frequently make more than $15K from it.
The folks applying to yCombinator are typically those that either don't want to be employees or have already been employees and decide they want to move on. They wouldn't even have a yCombinator app if they didn't have their own ideas. If you want someone to work on your idea, you have to compete with the hundreds of other Boston-area startups that are now hiring.
I'd also posit that when finding co-founders, you are in a sense finding someone that will help you execute your idea (and that makes them no less of an entrepreneur).