Perhaps an exclusive identity fails because it cannot sustain itself for long due to dwindling numbers of said exclusives in identity, meanwhile an expansive identity will always remain strong as no one is exclusive due to arbitrary reasons, but rather is based on ones commitment to the identity.
For example, for a person to be white is a fragile thing, evident since not even white people and agree who is white. This leads to fragility as all it takes is for a suspicion of unwhiteness to take the exclusivity out of someone. And with predicted intermarriage rates to increase in the USA, this will only further weaken its identity or change it drastically.