The articles says 'But the aim is usually to diversify risk, not concentrate power, by taking capital from the original family business and putting it into a widely spread portfolio.'
It uses the word 'usually' and it's also a blanket that's trying to generalize all these parties, while in a reality that may not be true.
Here are 3 constructed but plausible goals one may have: 1) Make 100% sure that you remain wealthy (make almost nothing but never lose anything) 2) 9-in-10 chance to do worse than SP500 index, but 1-in-10 chance to get 'literally take over the world' rich 3) Investing in causes that seem good for the world a la green energy, tesla motors, etc
As a counter example, I present cases of lottery winners who go broke. There are multiple documented cases of 10+ million and 100+ million winners who lose the majority of their wealth.
Some of it may be the forced diversification you get with this much money. It's hard or impossible to dump $100M into a single stock unless you are an Elon Musk type and own the company. So if you make a bad bet and lose a million bucks, well, that's just 1% of your portfolio and the rest will make up for it. Of course you're a regular millionaire and lose a million bucks on a bad investment you are going to the poorhouse.
You could dump $100M into GE without triggering anti hostile takeover actions, but they're also not likely to go bankrupt anytime soon.
Maybe some hundred millionaires could have had their portfolio completely tied up in Lehman Brothers a few years ago, but even then you probably would have gotten a fair bit of money back.
But this goes back to my point that as long as you aren't pants on head with your investments you are beyond the point where it's possible to ever be poor again.