You're still selecting for those with favorable startup employment experiences, so the problem still stands: excluding them does not give an accurate picture of the typical result if bad experience is what's typical.

Now the question is: when a startup fails, how often does it turn out to be a valuable experience anyway?

The most valuable experiences out of it are a bit of humility and growing your personal network. That's why this kind of employment IMO only worth it in the beginning of your career.

i think the hands-on, general experience at a startup far outweighs the silo'd, specialized experience at a big company anyday

You're deep in tautology land. Of course people that had a good experience with startups had a good experience with startups.