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.
I couldn't tell you off-hand how many presidents I've lived through, how many prime minsters I've lived through, how many wars I've lived through ..
But I can tell you I've lived through one Queen.
Even just logistically, to replicate this takes a young start that's getting less and less likely. If we assume Charles has 10-20 years left on him, that'll make William 50-60.
The oath itself wouldn't stop the British Parliament passing an Act of Parliament to abolish the Crown and replace it with some other system. In theory, the monarch could refuse to give their assent to the proposed law but given that would cause a constitutional crisis, in reality the chances are the monarch would assent and the system could be changed.
It seems it would take a republican government in power _or_ huge public demand that the monarchy to be abolished for that to happen which seems unlikely any time soon assuming King Charles III and his successors don't err massively.
At this point it's just a real estate company with an opera front, at some point the brits will wake up and see it for what it is