Having a large ranch of data cows is one of the ways of getting there.
https://www.corpmagazine.com/shirt-sleeves-to-shirt-sleeves-...
Compound interest only works if you can maintain your returns without going negative. Even ignoring death & family members, the world changes around you and your mental models become outdated. A hundred years ago, investing in steel, oil, and automobiles was a sure path to riches. Now, not so much.
https://qz.com/694340/the-richest-families-in-florence-in-14...
The Rockefellers are still the 24th-richest family in the US: https://www.forbes.com/sites/kerryadolan/2016/06/29/billion-...
Sure, it takes work keeping up - the Vanderbilts are a great example of what not to do - but money does usually beget money.
Carnegie is, I think, the most interesting example: He gave away almost all his money. His belief was summed up with "The man who dies rich, dies disgraced".
The Carnegie Corporation (a foundation), however, held on to the money neatly - their current endowment is $3 bln.
Just because you can sell a company for that much money doesn't mean it's not a lot of money
As a sibling poster said, that's a pretty sweet deal in exchange for... nothing aside from the luck of being born to the right family.
Many dystopian novels have explored that concept where at the point people can be kept alive indefinitely the moving around of wealth stops. Anybody not in the 'club' at that point is doomed to fight for fewer and fewer available resources. I personally don't feel that is the most accurate prediction of what will happen when billionaires become immortal but I recognize it as a possibility.
I'm more inclined towards the second, simply because even if the rich don't change, the world around them does. Joy's Law: "No matter who you are, most of the smartest people work for someone else." In my view, most of the rich got there because they had mental models that were particularly well-adapted to the times they lived in, and life circumstances that let them capitalize on the opportunities that this let them see before anyone else.
I've noticed that even in my relatively short life (I'm mid-30s), a lot of my mental models have already become outdated. When I started my career, desktop apps reigned supreme, you shouldn't even bother competing with Microsoft, and the richness and responsiveness of your UI determined how many customers you got (well, that and having a Windows version that was all of those things). By the time I graduated from college, this mental model was already out-of-date - and the people who recognized it was out-of-date, Larry Page and Mark Zuckerburg and Jeff Bezos, are now literally among the richest people on Earth.
I can only imagine how out-of-date someone's mental model would be after 1000 years of being on top of the world. Could you imagine putting Charlemagne into the modern world? He'd be dumber than the most idiotic teenager.