You say that time as to pass before a book can be considered a classic,don't you?
I think this way of looking at the problem can be misleading, if not at all incorrect. The lapsing of time can add value to a classic book, in the sense that more gneration can find shelter in tis pages, but this work often in one way only. If a book hasn't the strenght, depth, beauty, to be a classic, it will never be one. Not even at the end of the world
It may be a very good book, but not a "classic". No matter how much time is passed.