^^^
I do think there are some styles that are objectively classic. Obviously there are brand "classics" that are associated with a certain designer and have longevity in a line. However, when I said classics in the sense of this thread (more like classics in contrast to trends), I was using it more to mean an item that doesn't look dramatically out of place in any decade or setting. The Kelly is a good example...it's basic enough that not only can I picture Grace Kelly carrying it, I can picture it fitting in well with an outfit from the 70s, with a business woman in the 90s, and with contemporary fashion. It's easy to imagine a 21 year old rocking the bag, as well as her great grandmother. No matter how much you like the Balenciaga look, it's hard to imagine Grace Kelly sporting one in her typical attire, simply because it's too distinctive and defined of a "look" to be a true chameleon. In my opinion, the bags that are understated enough to truly blend into almost any era or outfit are the bags I would label "classics." Birkin or Speedy type bags are simple, so instead of the bags projecting a look, they adopt the look of the outfit it is paired with.
That is what I consider the line between bag trends and classics. If you have to match your outfit to your bag, it's probably trendy. If your bag adapts to your outfit, it's probably classic.