I'm not arguing your idea that there is a point where you no longer can find anything to spend money on. However that point is different for different wealthy people and I don't know and you don't likely don't know either where your limit is or my limit would be. It would not surprise me if I personally had the money and decided to buy a yacht and then later sold it when I realized that while it sounds good I wasn't actually using it. But I don't know. Maybe if I had the money to afford a nice yacht I would use it all the time. Similar for those mansions. Would I live in a mansion or would I buy one first test symbol and then realize I didn't actually care and downgrade? I know someone who has made a lot of money remodeling the mansions of rich people who have decided downsize. For them downsize includes a master bedroom that is bigger than my entire house. And there were other bedrooms in that mansion, but it is still a down size.
I can't argue the point as well as educated economists can. I'd like to point you to Gabriel Zucman, for instance. What really got me in the end is this: he proposes a 2% wealth tax. It's not a number out of thin air, he actually reasons very well how he got there. You know how much the average billionaire wealth increased over the last years, annually? It's 6%. So it's not even suggesting "you need to have less". It's suggesting "you should accumulate more a little slower than before".
Also, sorry, I really can't emphasise with people who got so used to having a yacht that they cannot imagine life without one anymore. It's peak consumerism, individualism, capitalism. My take is that we need to rethink _some_ things.