I remember this movie from my childhood and it still doesn't make much sense to me. Perhaps it is because I have only seen the first movie, but it seems to me that the concept of a 'story' that never ends has limitless possibilities. Therefore, they should still be making movies. In fact, I rather like that idea.
The problem with movies based off of a series is that the audience inevitably wants to see recurring characters. Unfortunately, if those characters are meant to be young, you can't really stop the aging process of the actors and there is a time limit to producing movies. But the Neverending Story strikes me as unique, much like Dr. Who. The Doctor can be anyone, he changes appearance and personalities when he reincarnates, therefore he can be someone different each time. He may have had interactions with the same creatures in the past and therefore has a ton of knowledge to draw from his experiences, but can still be surprised by new experiences.
The Neverending Story has the possibility of continuing a tale centered in this alternate book reality. It doesn't need to focus on the same characters or the same concepts. There's always the danger that the book may be put down or misplaced for too long and the world can come to an end. Since the characters are aware of the reader and can even interact with them, they can make pains to limit the amount of time the book gets shelved or plead with the reader to pass it on if interest has wavered. Even so, their existence can continue with other conflict, making it interesting for the new readers. Heck, a new reader can add to the society with their imagination, so there can always be new characters, new plots, and new places to explore since the book is what the reader makes of it.
And yet, the movies came to an end at some point. It seems like a waste of a concept and false advertising.