Why the OT worked so well is that the movies are essentially guilty pleasure movies that are fun as hell to watch, yet they define the SciFi/Fantasy Genre because there is more to them if you dig alittle bit. The OT movies do have a good story to tell, do have depth to them in that there is an overall arc, do provide humor with lovable characters to keep the movies from being TOO serious, have a blend of special effects and action to keep the movies from being too slow, and also provide enough drama that makes the movies enjoyable for adults cause they have those 'it' moments or goosebump moments. That was a formula from 77-83 that worked to perfection for Lucas, and I believe that is why the OT movies were ALWAYS going to be better no matter who wrote/directed the PT.
The PT in theory is a drama, cause a tragedy needs great drama to capture the bitter end and how everything ends up. But you watch Jake Lloyd/Jar Jar Binks totally make TPM laughable, you have a cruddy romance in AOTC that has some of the most cringeworthy moments ever put on screen, and then you many attempts at real drama in ROTS totally fail, especially near the end cause Lucas chose an average actor like Hayden to carry the film. Just think of great dramas: Platoon, Rain Man, Schindlers List, etc, all of them are well acted, well directed and provide that emotional punch that really grabs the viewer, a SW film is not made to do that, so in a sense it comes off totally distant from the OT. But does anyone here want to watch Schindlers List 300 times, even though it is one of the greatest movies ever made? No, it is not a fun movie to watch, it is serious, and sometimes you need time to watch it again, that is what a powerful drama does. A SW movie is just a fun 2 hours that you can revisit anytime you like.
The OT worked cause it didn't take itself too seriously, yet it still didn't cheese it up like movies Independence Day, where that is the ultimate guilty pleasure. The PT took itself too seriously, and in the end comes off as less dramatic as the OT, and IMO, it never had a shot.