To put it more succinctly, these original stories work because of the specifics of the plot. The universe is just there as a kind of generic backdrop, because it's about outer space and outer space is huge. It keeps the main story from feeling like its operating in a vacuum.
But beyond the Skywalkers/Kenobi/Yoda, lightsabers and the force, the Star Wars universe is just as generic as any other sci-fi universe if you really look carefully. Even lightsabers themselves were not at all unique.
Randomly designed ships? Check.
Vaguely different looking blasters? Check.
Fictional money? Check.
Exotic planets? Check.
Lucas' genius was in using certain generic elements to reinforce the story he was telling. The walkers on Hoth are generic until you realize they're a metaphor for the Empire's brute force tactics. They are powerful, but big and slow and don't pay attention to detail. It's why Luke was able to destroy the death star and why he was able to take out the walker. Taken out of this context, the walker just becomes a generic stomping machine you might find in any other generic sci-fi movie. In Star Wars, its function services the plot, which itself is already brilliant. And they were scary because you were already so invested in the characters. Examples like the walker are just one of the many little things that make the movies exceptional and stand out from the pack.
I'm not convinced the new movies have that brilliant spark. TFA certainly didn't convince anybody of that. The new trilogy's main attraction is finishing the Skywalker story. Everything else is basically generic fan fiction. Rogue One was the perfect example of taking SW iconography and telling a lackluster story. So when I see Disney carving out a Marvel type business plan, I just roll my eyes. Marvel has tons of brilliant storylines already mapped out from decades of comics. Disney is basically just making things up, but as I said, Lucas is what made that universe worth watching at all. Without him, it's not that interesting.
My point is a bit philosophical in nature, so I don't expect people to totally agree with it. But to me these stories use the universe as a worldbuilding exercise to reinforce the main story and give it scope and metaphorical context (like the walker sequence), not invite more delving into that universe because it seems cool. It's purely functional. So using the name Star Wars or Alien becomes virtually pointless to me. It's not that it's off limits, it's that I don't understand why I'm supposed to be excited about it. Honestly, I'd like to see Johnson be able to funnel whatever great ideas he has into his own stories, not continue to play second fiddle to the Skywalker story and further Disney-fy the franchise.