I get that there will be some people for whom that is an attraction, but given the vanishingly small chance to see the Queen outside of particular public events, I think you can safely chalk up the impact, outside those public events, as being nominal.
The potential added tourist value in being able to fully open up access to Royal sites (not to mention the value of repurposing some of them, as done with the Louvre) has to be considered as well, and I can't imagine how it wouldn't offset the lost revenue by many times over.