From The Making of Return of the Jedi, page 351:
Whether or not you think the Making-of books are skewed, they're certainly thorough, and there's no reason not to at least give an offhand mention to the books if they truly had an impact on Lucas's decision to move forward with more films.Following the groundbreaking effects that ILM created for Jurassic Park—as supervised by Muren and the result in part of Lucas's ongoing investment in digital technology—tentative steps began for new Star Wars films. Lucas had recharged his batteries and was prepared to step into the fray once more. The first move was a special edition of the Star Wars trilogy, which confirmed the world's ongoing love of the franchise. "I'd expected Star Wars would go away," Lucas would say. "The popularity of the special edition was a revelation."
I'm aware that the public popularity had waned post-1985 with no new movies on the horizon and subpar spinoffs. You could be correct that Dark Empire and Heir to the Empire spawned more products – they absolutely spawned more comics and novels – but giving them credit for reviving the Star Wars films or convincing Lucas of SW's popularity seems to be going a little too far. Other than using a few names or character designs in the films (yes, I know about "Coruscant"), has Lucas ever showed signs of caring about the novels? He's always treated them as a separate entity, regardless of what the company line has been. If you can find a quote of Lucas giving credit to the books, I'll believe you.