I don't think that we can really say that the Harry Potter series will never be classics. A book becomes a classic by virtue of still being read long after its author (and the original generation of readers) are long gone. Mark Twain once said, "A classic is something everyone wants to have read and nobody wants to read." While I doubt that Mr. Twain meant it literally I can see some of what he meant. Classics tend to be long in a lot of cases, and take time to read through. Certainly none of the Harry Potter books are short.
And I wager some of the books written in the 19th century were thought to be 'classic' worthy and yet are long forgotten while others disparaged as 'popular' fiction at the time have remained in the public eye.