For you to say it’s derivative it needs to be a derivation of something else, I would like to ask you what exactly would that be?

Someone who grew up playing D&D and similar might watch Lord of the Rings and say it’s derivative, without understanding that everything he knows is a derivation of it. There’s a quote from Sir Terry Pratchett about it:

J.R.R. Tolkien has become a sort of mountain, appearing in all subsequent fantasy in the way that Mt. Fuji appears so often in Japanese prints. Sometimes it’s big and up close. Sometimes it’s a shape on the horizon. Sometimes it’s not there at all, which means that the artist either has made a deliberate decision against the mountain, which is interesting in itself, or is in fact standing on Mt. Fuji.

And Dune is exactly the same except about space empires.