There's no reason why further Star Wars films can't be made. The exact decision is what exactly to focus on. There's enough alternative media out there to show that the Star Wars galaxy has a depth to it that could provide plenty of material in some shape or form that wouldn't require Lucas (or whomever was handling these things) need to go anywhere near the characters and stories that are central to the existing six Star wars movies.
In fact, this is the route I'd prefer to see. I think further movies featuring Han Solo, Princess Leia, Luke Skywalker and the rest would be a huge mistake. All the loose ends have been tidied up on all story threads, so there's no further need to follow these characters, well loved though they are. Just like Tolkien's Middle Earth, the Star Wars universe is a goldmine of potential characters and stories as can be attested by the wealth of merchandising out there.
The PC role playing games series "Knights of the old Republic" take place 4000 years before the events of Lucas' films and still retain the character and atmosphere of the films. There are a few older first person shooter games ("Jedi Knight") featuring a character called Kyle Katarn (if I recall correctly) who could easily step forward as a new, lead character in the familiar Star Wars universe. I've not read any of the many existing books, but I'm led to believe these could also provide a lot of interesting plot ideas.
My own personal opinion is that it's a no-brainer. LucasArts is a business and Star Wars is its most successful franchise. While I'm aware that George Lucas is very reluctant to hand the reigns over to someone else and demands a lot of creative control, it could be possible that Lucas could be directly involved in creating characters and plots for future films (or even computer games, novels, cartoons, television programmes etc.) and then letting other people go off and flesh these out into detailed scripts and storyboards. Perhaps then, the Star Wars franchise would once again open up to everyone and more mature films/games etc. can be created alongside the child-friendly aspects that Lucas seems to favour.
From my own perspective, that's the ideal solution.