I can see that "five-year anniversary" is redundant, once the etymology is
pointed out, and that, for political (and maybe justification of
employment) rather than linguistic reasons, it might be adviseable to use
the ordinal form. But in a larger sense of the way most people use a
language, why refer to etymology? If for a significant number of people
"five-year anniversary" is acceptable, probably because Latin is a dead
language, what's the problem? I love English as much as the next man,
mostly due to being limited by congenital stupidity to the use of this one
language. However, if I were a real linguist, I might start a campaign
to outlaw redundancies such as "Avon River".
This discussion is very interesting, but I must admit that I feel a sham
by bringing the blazing light of the finest linguistic minds and
terrabytes of empirically researched data to bear on a point that in an
EFL context ultimately serves only political ends. :)
John