To me, when a bag becomes cemented as representative of a brand, that bag should ,well-deservedly, be considered a classic. You see that with the Dior Saddle (making a nice big comeback), the Balenciaga City, the Chanel Classic jumbo, the LV speedy...and to a more modern sense, you see that with the Luggage bags - there is no other line of bags produced by Celine that is so easily identifiable as quintessential Celine. A classic to me, is something that even my purse-hating SO can identify as by X brand.
Trends change - like whatever Chanel seems to love produce as a lower-end "classic aspiring" piece - the riviera, the 3, the shiva, the Business line (flap, affinity, backpack) etc. To me, these rotating bags are trends, the only thing "classic" about them is the use of the quilted leather.
Also, honestly, buy what you love, worry less about trends vs classic pieces, if you like it and must have it. Get it.