A big part of what has made
Star Wars so huge is the overwhelming visual-effects spectacle of the films and the picture-worth-a-thousand-words effect of its actors. Even if Lucas were a literary genius, recreating the backdrop of something like the Mos Eisley cantina or Jabba's palace would be a formidable task.
Without the movies, the mass appeal to children would be less (so no enormous production of Star Wars toys and merchandise)
One might compare the impact of our hypothetical novels-only
Star Wars to other fairly successful SF novel series of the '70s, '80s, and '90s. The examples popping into my head are somewhat more recent (Bujold's
Vorkosigan series and Weber's
Honor Harrington series), but they give you a benchmark for the level of cultural impact the series would be likely to have. Well known among science fiction fandom, with quite a lot of readers overall, but nearly no mass market penetration.
And of course, there would be
endless debate over how or if they'd handle movie rights. What, the destruction of the Death Star? As a movie? It'd never work!
Unfortunately, based on historical precedents there's a good chance the movie would never actually get made.
...
Another thought, we'd probably have seen all nine 'episodes' novelized by 1990 at the latest, and the other novels would be... interestingly different... from the plots we actually have for Episodes 1-3 and 7-9. Especially 7-9; we may never find out what Lucas originally had planned for those.