The thing is - much as I love the movies, mostly because of nostalgia, they really are awful, in many ways.
SW: The acting is wooden at best, and terrible most of the time. Harrison Ford might be a movie star, but as an actor, his range is… shall we say limited to sullen or glowering. It really doesn’t matter what happens in the cantina scene, because it’s extremely obvious that Ford is waiting for his cue
The same in the big garbage room, every one is standing around, ready to grab whatever they used in the previous take and re-do it for the umpteenth time. Mark Hammil can’t act, Carrie Fischer can’t act. Sir Alec can, but was given awful material to work with. The whole thing is cheesy beyond belief and had it been released today, it’d sink like a stone at the box office.
TESB: It’s without a doubt the best in the whole franchise, maybe because Larry Kasdan had a hand in the script, maybe because they realized how limited the actors were and wrote a screenplay that didn’t demand too much from them. Yoda is kinda cute, but… it’s really to close to the Muppets for comfort.
RoTJ: After the peak in the second installment, it went downhill. Fast. Lucas was running out of ifdeas and recycled a lot of stuff from SW. The deathstar of course, but there is basically not enough story to fill a two hour movie, hence the long intro where they’re saving Han. Sure, we got CF in gold bikini and the fight above the giant, fanged, vagina was kinda good, but it didn’t advance the storyline one bit.
And then, there are the damn ewoks. I remember a rumour going round that Lucas knew that Spielberg got burnt on the merchandising from E.T. The dolls were really ugly and didn’t sell as expected. Supposedly, ewoks were added so that there would be cute and cuddly stuff at Toys’r’Us.
Looking at what Lucas has done as a filmmaker, on his own, it’s no surprise really that the three prequels are getting so much venom. Lucas is a talentless hack, who has had two ideas in his whole life: Making a movie about his teen years and recycling old space opera he grew up with. His on and off movie making partner, Spielberg, is also a hack, but he has talent and, even if I think he fails, an urge to make movies about more important stuff than space droids and giant ducks.
So, to get back to the OP, Lucas could never improve on the original, because it’s dreck to start with and he doesn’t have the talent to remake and improve them. Now, another director, a better writer and some good actors might make it. Lucas will never let that happen, though.