Earlier quoted context omitted.
If the royalty went away and the UK took possession of its lands, castles, and holdings, it could make a killing off tourism to those private or semi-private residences. Other European countries that are no longer monarchies do this just fine.
This is the point that people who say "but what about tourism?" completely and spectacularly miss. Versailles is one of the most visited places in the world. Having no royal people in there makes zero difference to that. We could have the tourism without the actual royals, 100% no problem there
1. Versailles has a boatload of art in it and is massive.
2. Some rather dramatic things happened there.
Buckingham palace has plenty of art, but so does the V&A Museum.
And you can't re-create the historical drama and preserve stability.