I'll refrain from speaking for everyone else, but personally I've been to London maybe ~20 times, and don't think I've ever seen Buckingham Palace up close. Like I'm sure there are people that are interested in the royal family, though whether those extend outside the UK and the former colonies, I'm not so sure. I'd wager that if the palace with the whole family just disappeared, its effect on the tourism in London would be percentage points, but since my personal view is probably skewing that downwards, let's say a few percent.