Except his film is just as derivative of the OT as TFA or RO, he just decided to recycle two movies into one.
Hang on a second. I think there's a difference between something that's "derivative" and something that's "iterative."
I see a "derivative" film as one that recycles elements from previous film(s). And that's it, really. Derivative doesn't necessarily mean "bad" either. Lucas' original Star Wars was
highly derivative of several different films and film genres, forming a pastiche of references to other movies, while also doing something really interesting and fun with them.
That's different from an "iterative" film, which basically re-tells the same story that was told previously with only minor tweaks. Even an iterative story can still be interesting, although it does grow stale after a while.
I'm not sure how familiar you are with it, but the Gundam franchise has been surviving for decades on iterative tales. They've tweaked things here and there, they've updated some stuff, but a whole lot of the stories that have been told within that franchise have been basically a recycling of (1) the original 0079 Gundam, (2) Z Gundam, and to a lesser extent (3) Gundam Wing. The newer versions are still interesting for the new design aspects, the new animation styles, and the new characters, but it's reeeeeaaaallly easy to spot the analogues for this or that character, and not just because so and so is the designated newtype (think psychic pilot, basically), or the designated Char clone (think Vader stand-in).
I see TLJ as derivative in that it's playing within the same universe and touching on many of the same themes, but it's also pretty out there in terms of how it upends convention and expectations that have set in for 30 years. There are surface details that are the same as what we've seen before, but Johnson's film does not (A) flow easily into an ROTJ-like resolution, nor (B) handle its characters the same way as before with the OT or PT. Sure, Ben/Kylo is a surface-level analogue for Anakin/Vader, but I see his motivations as being completely different and in many ways more interesting than the Anakin we see in the PT. (Vader's less of a character in the OT and more of just a threatening monster, until you get to the end of ESB and learn more of his motivations -- which don't really match up with PT-Anakin, either.)
I kinda feel like the main point of TLJ is the Rey/Ben relationship and their respective relationships with the Force and what that all means. That and introducing some of the new themes that open up the possibility of who can be a hero in this universe which, while new in some respects, seem to me to fit the setting quite nicely.