The point is Historical Significance
It makes sense but I don't totally agree with you. Diana never fitted into royal life, she felt rejected. She always said that she indentified herself more with people in lower social class, she is by being burried in Althorp : everybody can pay her a visit. Her worst fear I think was to be kept away from them, she WAS the People's Princess after all.
Princes William and Harry are big enough to tell what they want to do. If they wish to visit their mother's grave, they can and as it was said previously, if King William V wants to move her grave, he can.
Althorp is her roots, where she was a lady, not a princess but still she had this special gift to help people. I prefer to see her burried in Althorp because we know that visitors come for her, not for the place or other graves like it could have been the case at Frogmore. At Althorp she is no VIP, just Lady Diana Spencer, one of the people.
Diana's Historical Significance lies in her having been a member of the Royal Family whether she felt like she "fit in " or not. At her death she was Diana, Princess of Wales. She was the mother of a future British Monarch.
In the first place, being buried at Althorp does not identify one with the lower classes.
In the second place, there is no such title as The People's Princess. She was Diana, Princess of Wales.
One's place of burial can add importance and significance to one's place in history. It secures their memory as among those who were important. For instance, if one were to be buried at Westminster Abbey it would be assumed that they had "Historical Importance."
It is my understanding, that the Queen was willing to bury her at Frogmore. The Queen understood her significance as a future Monarch's mother and as Diana, Princess of Wales. Earl Spencer acted like a child throwing a tantrum and proved himself to be a HISTORICAL pinhead by not securing his sister's place of importance in the Royal Family. She may not have been styled HRH any longer, but she certainly had a place among the Royal Family. She is Prince William's mother. She had not remarried. Her title was still Diana, Princess of Wales. Earl Spencer treated her as a "Pop Princess", not a Real Princess.
Forty years from now not many will remember Princess Diana the way she is remembered now. Those who remember her will be old and many people will not have actual memories of her. Placing her at Frogmore, however, would be evidence of her significance. Earl Spencer devalued her historical importance by burying her at Althorp.
She was no longer Lady Diana Spencer, but Diana, Princess of Wales, mother of William V. She belongs at Frogmore.