The exception shows there was no rule. The "rule" was "code is law, up until 51% of the network decides it isn't". If your transactions can be wiped out by a fork leaving you on a dwindling/minority version of the transaction history, you fundamentally cannot trust the system. "We only did it once" doesn't work, either, because if they'll do it once they'll do it again, and no amount of bombast will convince me (since, after all, they made bombastic assurances before they decided to make the first exception).

Also, "the exception proves the rule" is a misinterpretation of the phrase. "Proves" here means "puts to the test", and the saying means that you can tell whether a rule holds up by seeing what happens when it is tested. Ethereum demonstrated that the rule does not hold up.