The thing about the OT is it is timeless, and that is key to fans sticking with a movie long after they have grown up. For me, growing up in the 80's, I loved SW, but I also loved movies like The Karate Kid. 20 years later, I see ROTJ as a good sequel, but not great, and I watch it more as closure to my beloved trilogy. SW & ESB are still classics to me, because they are great movies, and great movies have the quality to withstand the test of time that good movies you may have loved growing up don't.
The Karate Kid is a perfect example of movie that has not passed the test of time, and I only watch it for nostalgia. As a 12 year old kid in 1984, I loved the movie, and watched it a zillion times on HBO. About a year ago I saw it on TBS, and watched it for the first time in about 10 years, and wow, it was cringeworthy to watch. I still enjoyed it, cause I thought of memories of 6th grade, but I won't be buying it on DVD and watching it as much as the OT.
Now for the PT fans who supposedly love them now, what will they think in 20 years? I don't doubt they love the movies now, but they will grow out of them, especially TPM & AOTC cause they are not great movies. ROTS seems to be the prevailing favorite around gushers, and I know my 8 year old nephew only likes that prequel movie, so that may pass the test of time, only cause every plot point is thrown in by Lucas. But do you really think 20 years from now, a majority of the PT gushers are going to pop in TPM out of true love of the movie? Do you think they are going to look at their movies and say, "AOTC, that is what I have been dying to watch lately, that movie is a classic!"
I still do that with SW & ESB. Those movies are so good, the quality has lasted, that I still yearn to watch them cause they are great, not because I was a big SW fan growing up. ROTJ is one movie of the trilogy that I look at as part of the story after watching two classics. There is no way fans are going to watch two average or bad prequels to get to the perceived great one of the trilogy in the PT.
The PT in 20 years will be like Star Trek fans, a niche audience. Those fans will be diehards, I don't doubt that, but it won't be any of the size that the OT still has these days, cause many fans are going to grow up and realize Jar Jar is ridiculous, and the love story in AOTC is atrocious, and what is left to them, one good movie, and that, a trilogy does not make. Trust me on this guys.