Very interesting, as always, but I disagree. I think Diana, and most modern royals, will be minor footnotes in history.
I don't think you can reasonably compare Diana to Catherine of Aragon. Catherine of Aragon unwittingly changed the course of western civilization. Henry's break with the Catholic Church had an immediate and significant impact on the lives of his subjects. It also significantly impacted political alliances with people in many other European countries, which also affected the lives of people all across Europe. Those changes still reverberate today.
I know there are a lot of Diana fans on this site, but Diana really didn't have a significant impact on the daily lives of the general public. Many people adored her but how many people's lives were significantly affected by Diana?
That is true of all modern royals. The average member of parliament have had more impact on people's day-to-day life than virtually any current member of the royal family, including the Queen. My understanding is that most people in Great Britain support the monarchy but rarely think about it.
Yes, Diana raised money for great causes, but I don't think that alone makes her a major historical figure. Prince Charles has raised a lot of money as have other royals and celebrities. Frankly, it's hard to judge how much impact the funds raised has really had on most people's daily lives. I am not aware of any disease that was cured because of Diana's fundraising.
Some people claim that she made an impact on the anti-landmine campaign. As someone who worked on that issue throughout the 90's, I can tell you that the campaign to ban landmines was well underway before Diana got involved and no country signed the Ottawa Treaty simply because of Diana. For those who disagree, please cite an example of a country who changed policy and signed the treaty after Diana got involved. The British Labour Party had announced its support for the accord almost a year before Diana got involved.
You can make an argument that she will be a significant part of royal history but, I'm not convinced that will hold up over time. She will be an important and studied historical figure if the monarchy ends with either Charles or William. But if it survives, the impact of her actions will be less and less important over time.
Essentially I don't think the royal family changed that much solely because of Diana. Certainly she hastened some modernization, but most of the changes would have naturally happened over time. For example, I have no doubt that William and Catherine would have openly lived with each other before marriage even if Charles and Diana had been happily married. Society changed since the 1980's. The royal family has always adapted to changes in society. That is how it has survived.
You raise some really good points, and I think ultimately that our difference in opinions is owing to the fact that we're considering history differently.
My comparison between Diana and Catherine of Aragon wasn't meant to say that they have an equal impact on history - they don't - but rather that they filled similar roles. Catherine was married to the King and was well loved by the people. They had a child, but owing to a failure in their marriage her husband strayed and sought divorce - in itself a huge controversy. Her husband then remarried, his second wife being a woman who was rather hated largely because she was blamed for her role in the divorce. In this regards, Diana is very much like Catherine - her marriage and divorce may not have the same lasting impact that Catherine's did (it's rather hard to have such an impact), but the basic role is very similar. And we don't remember Catherine simply because of the profound impact that her divorce had, we remember her (in popular culture at least) because of the controversy, scandal, and tragic romance of her life. It's something that appeals to the mass public, which is why it's so well remembered. Diana may not have the same lasting impact as Catherine, but she has the same lasting appeal. Hers is this story that is too easily made into this tragic romance.
Curryong raised another great comparison - between Diana and Caroline of Brunswick (wife of George IV). The story of Caroline and George may not have the same mass public appeal as the triangles of Catherine/Henry/Anne or Diana/Charles/Camilla, but the person that Caroline was and the role she filled is similar to that which Diana filled - she was married to the Prince of Wales, she had a disastrous relationship with her husband. She didn't really have any lasting impact - beyond the child she gave birth to, she didn't really achieve anything lasting (and even her daughter, in historical regards, was more important in what her death achieved than anything else) - but she's still remembered in history and still discussed today.
There is a theory that history is about Great (White) Men doing Great Things (as my grade 9 Social Studies teacher put it; being the subversive brat I was even then, I enjoyed spending an entire class coming up with the names of people who did things that weren't necessarily great, people who weren't necessarily white, and people who weren't necessarily men that history also remembers). This type of history tends to be more political in nature and talks about the huge events that shape the world. The events that are looked at in such a history tend to be the ones that have a lasting impact, the people are the big names that are remembered. So Henry VIII and formation of the Church of England, or William I and the Norman Conquest, or Adolf Hitler and the Second World War. These are the types of names that are remembered by the mass public, even if the masses don't always remember just exactly why they're important.
However that's one very narrow subset of history, one that's not even necessarily the most popular among academics. Expanding on the Great Man theory, there are historians who look at people who were "great" - the big names - but who themselves didn't necessarily achieve any great lasting impact. Consider some of the American Presidents - not the Roosevelts or the Kennedys in nature, but the lesser ones who came in for four years, didn't achieve much, and then faded into oblivion (I am sure there are some, but I don't know enough about US history to be able to name one... I could name a few Canadian PMs of the like though). They might not have had a lasting impact, but they're still studied by academics who look not simply at their lasting impact, but also at what happened while they were in office. The impact that they had during their lifetime.
I would argue that Diana is going to be a studied person not because she achieved anything of lasting importance - I agree with you in that a lot of the importance attributed to Diana is an exageration on the part of her fans - but because of the significance that she had during her lifetime. There is the chaos of her marriage, which is going to be studied for a good long while - especially as people who were alive during the events pass on and records that were made during her lifetime become available to the public. The diaries of people like the Queen Mother, the Queen herself, etc, aren't going to be available to be read by members of the public (even a purely academic public) until well after their deaths. There is also the fever with which the public loved here, the Diana effect. We might be at a point where Diana is no longer the big selling factor it once was, but during the 80s and 90s it was a huge thing. Academics are going to be studying that for a good long while.
I would also disagree with the idea that Diana no longer sells. Sure the Naomi Watts movie didn't do well... but it was also a really bad movie with a limited release. You still see Diana's name being mentioned in news articles - the conspiracies around her death, the recent talk about what's happening to her collection now that Harry's 30, the comparisons between her and Kate. She may not be selling now as much as she was in the 90s, but she's not an entity that has disappeared. Every time you see a headline about how Kate's pregnant with a girl and they're going to name it Diana, it's a paper trying to make money off of the Diana story.