Well, perhaps it is a bit silly, and maybe it's anachronistic, too. But I'm not 100% certain that it's wasteful. There's no question the trappings and palaces and crowned jewels and ceremonies bring in huge amounts of tourist money, which pays for quite a lot, actually.
But something else -- the Crown is something that the British people (perhaps less so Canadians, Australians and other Commonwealth countries) can gather round together, because it is non-political. What do you Americans have? Do you look to Trump or Biden as your rallying point? No, supporters of one hold the other in total anathema! The Supreme Court? Hardly -- it's as political as the Senate and House nowadays. The Constitution? Nope, one side is busily trying to tear that to pieces. The flag, maybe, but that doesn't seem like much in the end, and the South still likes waving around that other flag.
Sure, there were a few anti-monarchical protesters, but how many compared to the hundreds and hundreds of supporters lining the Mall in the rain for days for just a glimpse? Or for the military, who playing their part in the ceremonies got to see nothing of those ceremonies except each other -- did you hear the cheer in the garden of Buckingham Palace? They were more than enthusiastic.
Today, Rishi Sunak is Prime Minister, but tomorrow it may well be a Liberal or Labour PM -- but both of them report to the same Monarch, who does not interfere but retains the right to advise and to warn. That is powerfully symbolic, a testament that in a Constitutional Monarchy, the government doesn't rule the people, the government REPORTS to the King, who represents the people.
And speaking of wasteful, the typical swearing in of a U.S. President costs about $100 million -- and you do it every 4 years! The last time there was a coronation was 7 decades ago,, when I was just 5 years old.