Do you have an alternative metric? I suppose we could count jobs created. My experience in the Bay Area is that as long as exits >> (funding * 2), it means that there are non-investors making some money and going on to create additional opportunities (sort of the criticality threshold for entrepreneurism).
Number of customers? Years in business? Profitable quarters? Going from startup to long-term business? Uhh, military contracts awarded? (okay, maybe not..)
Are the majority of startups only ever going to be short term ventures?