I don't know that Diana was universally loved, really. She had incredible charisma, and she was very successful in crafting an image that fascinated people, but fascination is not the same thing as love. I think some people were in love with the fantasy of Diana--a lot of people, actually. But again, fantasy is not the same as the real person, who was much more complex, and, in my opinion, much more interesting than the cardboard saint/fairy princess.
I also think that William, Kate, Harry and Meghan are working very hard to not be the fantasy figures that Diana became. Diana gave a up bits and pieces of her privacy to craft her image. William, Kate, and the Sussexes seem determined not to do that.
I see your point and agree with it but I would say she was loved as you saw the reaction with her death. The younger royals seem very unapproachable. I don't know if thats a good or bad thing. At least it seemed like Diana was approachable.