I can't possibly imagine how the 'dark' ending would have been any better.
Han dying in the second act after they spent the first act rescuing him from Carbonite? "Yay! We've finally saved Han from Jabba, and now we can go on to defeat the...OH NO! THEY KILLED HAN!". Talk about a wasted first half hour or so.
Luke walking off into the sunset? What reason would he have for doing that? He's got a family with the Rebellion, both literally (Sister Leia) and metaphorically. Even if the Rebellion is in disarray, he could still stay around to try to help put it back together. The only reason I can think of for him to walk off into he sunset would be if he suspected he would need to prepare to deal with his Sister and her new role down the road. Speaking of which...
Leia struggling with her new responsibilities as queen? Queen of what, the galaxy? After they just finished killing the Emperor? First, how does that fit with her character at all (yes, she's princess of a dead planet, and she's been fighting with a group who's purpose is to restore the republic), and why would a rebellion seeking to overthrow an Empire go along with putting a new monarch in charge?
Basically, as I see it, the 'dark' ending turns not just the movie, but the whole trilogy into a shaggy dog story. Or worse, a shoot the shaggy dog story.
The 'light' ending, on the other hand, more or less wraps up the storyline of IV-VI. Yes, you can argue "hey, but what about all the other Imperials?". But can you argue with the ending providing us with an image of a galaxy who's future is looking up, and leave us with hope that everything is going to work out in the end? As long as you don't think about it too much, the end of Jedi leaves you with the expectation that everything is going to turn out fine.
Even if you DO think about it too much, the Imperial fleet just watched their superweapon get blown to smithereens - that has got to be a HUGE blow to moral. Not long before that, they watched their flagship, the Executor, crash into said superweapon. What has to be another big blow to moral. Then, you have both the Emperor and his clear Successor dead on said superweapon. With their flagship gone, I can imagine the possibility about there being a lot of confusion about chain of command. Palpatine's dead, Vader's dead, Piett is dead, so who's in charge of the fleet now? Who's in charge of the Empire now? It's a breeding ground for infighting and the Empire tearing itself apart, leaving the unified Rebellion with the advantage.
Finally, when I think about it, the 'dark' ending actually grants more opportunity for further merchandising than the 'light' side. The 'light' ending pretty well wraps things up. It ends the trilogy, makes it more difficult to get a good entry point for a sequel without it feeling forced, which means without the movies, he has to go a different route to keep the tie-in merchandising going. The 'dark' ending, on the other hand, leaves open a perfect cliffhanger for a fourth installment, making it much easier for him to keep selling movie tie-in merchandise.
So if the light ending was chosen to sell more toys, then I'd say Lucas didn't think things out very well. Considering his foresight with things like getting the merchandising rights for the first movie, it leaves me a tad suspicious about the original claim of the article.
Some people may not like the way the movies turned out, but all things considered, I'm more or less willing to go along with accepting that the movies more or less fit with his vision. You may not like his 'vision', and perhaps there are times when someone could have stood to step in and say 'hey, maybe this isn't the greatest idea', but that doesn't mean Lucas is entirely driven by ulterior motives.