I must respectfully disagree. My Random House unabridged dictionary (first edition, copyright 1966) lists as the second definition of "
Cassandra": "one who prophesies doom or disaster". I think that if it makes it into the dictionary one can safely say that
Cassandra now has that general meaning and that English speakers can use it in that way without any direct reference to the particular
Cassandra who lived in ancient
Troy, in the same way that we now "go Dutch" to a restaurant without consciously realizing that was originally a reference to a particular nationality.