In my opinion RW one of the great functions of good writing, now sadly often in decline (though it is making a comeback), is exposing the reader to new (as well as ancient) terms, information, historical references, metaphors, allusions, etc.
Before modern communication modes readers were often exposed to new information through the medium of the novel. When I was a kid I often increased my vocabulary through reading novels.
The author was well educated and used his novel not only as a vehicle for entertainment, but for various other purposes as well, to make an important philosophical, social, spiritual, or psychological point. Part of that function was to expose the reader to terms and ideas that many might find obscure, or might never otherwise be exposed to. I've always thought this was an important part, or function, of fiction. I'm sorry to say that I think fiction has too often become synonymous with "mere entertainment," but I think that is but one function (and probably not even the most important function) of fiction.
But as the others have pointed out the internet allows easy access to information. So if a reader didn't know the definition of a leitmotif, it wouldn't take long to track it down. (Or one could just do what I sued to do, keep a dictionary and thesaurus nearby when reading - reading and writing are after all more than the art of "static word play.") And if the reader didn't know the definition, and looked it up, and learned the definition, then it certainly wouldn't harm them, and at worst might improve their vocabulary.
My personal opinion is never talk down to your audience. A novel is not a comic-book, is should be a work filled with "new ideas." Assume if you're audience doesn't know a thing, that they're smart enough to learn or to want to learn, and that one of your functions as an author is not just to entertain but also to inspire. Even to inspire one to learn what he doesn't already know.
I'd keep the term.
Exploit it even.