So, just to elaborate from my earlier questions...
It's certainly very nice to think of all these lovely homes and gardens and grounds and relics and artefacts and art and so on as being nice things people want to see, especially as Liz Two seems lovely and drives a Land Rover and stuff... but...
Every single one of them is derived from or built on conquest and slavery. Their history is written in free-flowing blood of almost anyone except the people who own them. For close to a millennium, the English blasted through Europe - on foot and then in ships too - killing people for any approximate combination of we liked the land, their prince said something rude about our princess's tits, and religion.
We weren't exactly unique on that front - France and Spain did it too - but stuff like the Crusades have a lot more in common with Nazism than they don't have. All of these places and things are relics of that era (as is the monarchy itself), and the fact that they all remain in the hands of the inbred descendants of the Hitlers in Hats who perpetrated it.
In addition to all of that, just think of all the utterly unusable land we have - in a country which doesn't exactly have a lot of it to spare - because a royal castle and its sprawling grounds sits on it and you'll get shot if you set foot in it. They're like golf courses with police firearms protection.
That naturally leads to the question of why it merits preservation at all. Certainly I don't want to see these castles bulldozed to the ground and replaced with a bunch of dreadful cookie cutter new-builds - and when you consider how many cities have these structures in them (or surrounding city centre!), it's unwieldy - but I don't really see the unquestioning preservation argument for monuments to Middle Ages brutality, genocide, rape, and pillage built by the slavery of serfdom.
However, accepting for the sake of argument that they do need preservation, we roll into the how. Once they're removed from the hands of the no-longer Royals, how do we even preserve them?
Obviously they need to pass into the ownership of someone tasked with preserving them, but who and how will they be funded - and how will they be prevented from returning the relics to their rightful owners/creators or selling off patches of the land (should they even be prevented from selling off bits of the grounds?), related buildings, or the main structure itself? I very much anticipate "York Castle: Brought to You By Kia, Movement That Inspires" as a best-case scenario here.
At the moment, the Royals are very much the best options for preservation of the lands and property they "own", if preservation is what's desired.