I don't think it's sad at all. I think sadness is reserved for something unfortunate, ie her dying in her 30s with so much of her life ahead of her, or that she's someone we're related to that we have an active relationship with and will miss.
For a person none of us know or have any kind of relationship with it's to all intents and purposes just a person who lived to almost 100 dying and there's nothing sad about that, rather something to celebrate that they were able to experience decades on this planet that most of us will not. And that's before you get to the potential murder, and helping obstruct sex crimes. Death at 96 should be celebrated for the amazing feat that it is and everything that person achieved.
I don't think there will be any changes to the monarchy, Charles has been wanting to be king so badly, and I reckon William will be right behind him when his time comes. I don't see anyone in the family bar Harry wanting to make real change and Harry isn't a prospect anyway.