My impression is that William benefited from just not being Charles, and some of the sheen rubbing off from his mother. Both of those things only go so far, and as he moves more and more into public responsibilities, he has more and more chances to bungle up. From the high of the early 2010s, the only way for him to trend was down, and its inevitable. William is, what, 40? Charles wasn't quite reviled when he was 40 too - he grew into that role.
Even if the monarchy isn't abolished outright before Louis or a sibling ascends, it's very possible that the United Kingdom in its current state may not. The unified crowns of England and Scotland may exist in title only, if that.