I agree, and we are probably witnessing a terminal decline in Great Britain, which may well lose its “Great” in the near future along with three of its constituent nations. We will therefore have to do more with less, while a population that is becoming increasingly restless and angry may be less inclined to pay for an extravagant Royal Family that doesn't appear to be doing its part.
I agree with your first sentence. It's the march of history. In 1066 and All That, which is absolutely brilliant in poking fun at the way history is thought about, they have 'Top Nations'. During 17th c it was France, then in the 18th/19th, Britain, followed by America in the 20th and now China (not Russia, I don't think). We aren't going to like it, but we can't do anything about it.
I'm a constitutional monarchist but I don't much mind how small the RF is. I would put money on William V definitely reducing the size of it. But I think it's beneficial to us, it's a system that works and has many advantages.
I think people are restless and angry but not so much about the RF. The instability and the perceived threats to Britain are translating into far-right voting. The corollary of saying that few people now care about the RF is that -well, very few care enough to protest or change things.
And frankly, why the hell would we waste time and energy on this question when the whole political scene internationally is so volatile. We'd be crazy to do it. Even the Labour Party know it's a dead duck. People do indeed have other concerns.