Because, like with mixing services, ZCash shielded transactions are opt-in and can easily be detected. So exchanges could decide to ban coins with mixing in their history or require extensive KYC information.

Yes it's a little different from the problem described in the post, but it's another facet of the fungibility issue.

> So exchanges could decide to ban coins with mixing in their history or require extensive KYC information.

The same way they could decide to ban Monero... seems like what you describe with z-cash is more complex to support in an exchange than simply removing a whole cryptocurrency, thus is less likely to happen.

Sure, but that's a different issue than fungibility.

Fundamentally not trustable.