Lucas to me has gone Vader crazy ever since 'I am your father' and though that is a great moment in SW-ville, it is when the saga became too narrow on the dealings with Darth Vader/Anakin Skywalker. I just dont want to see Anakin Skywalker as the star of SW for 6 movies now, and that is why the PT doesn't fit the OT.
As much as the OT was about Luke, it wasn't solely about him, Han & Leia were just as important to how everything occurs in the three OT movies. The PT is about Anakin, and if anyone disagrees, just check out the 9 year old kid in TPM playing Anakin, right there I saw that and thought, 'Character arc?" That is why Lucas says it is the tragedy of Darth Vader as the saga story now, it is the character arc of that character, and if you don't love that character, or don't find him compelling you aint gonna love the movies, no matter what quality they are.
Lucas should have given equal screentime in all 3 movies to Anakin, Obiwan, and Padme the same way he did with Luke, Leia, and Han. Even though Anakin is the star of the PT, his turn to the darkside should have been a plot point, not a turning point, or a build up of 3 movies as Lucas wrote it.
He wrote the first two movies in anticipation of Anakin turning, that is why they seem like setup movies now, cause they are! Once Anakin turns, the real story of the PT happens, but the problem is there is only an hour left in the trilogy, and all the good shit is either tossed or put in a montage to music like a greatest hits.
Sure we need to know certain things from TPM & AOTC, but they plod along for 4 1/2 hours of EU stuff that is nice to read in a novel, but not in the movies. ROTS should have been the PT story, and not just Darth Vader running around killing people as Lucas accuses many of us of wanting, but the Clone Wars, the birth of the Rebellion, the Jedi Council all should have been more prevailent in the main story rather than just plot points, or having to watch some cartoon to get more info.
Lucas put the scope too much on Anakin, and it started in ROTJ with Vaders redemption, but turned out pretty good, cause it was a subplot to the Luke and the Rebels story.
The future of the saga 1-6 will not come down to quality per say, but do you want to see 6 movies about the story of Anakin Skywalkers rise, fall, and redemption, and I believe that is a harder story to sell to the masses then the uplifting story of Luke, Leia, and Han. That is why I believe you will have a diehard niche fanbase of saga lovers, but not nearly as big as the OT crowd cause it is too narrow of a story. I wanted the PT to be a backstory, not a linear story, just a telling of the events that happened before Episode IV, but didn't totally need to marry the trilogies to gain its value. For me, whatever we say about Jar Jar, and midiclorians, I am not interested in 6 movies about Anakin Skywalker, it is just not that interesting.