To keep up with the index is probably really hard.
and sorry, i misremembered the numbers. he got over $400 million (at least) from dad, and that would be worth about $2 billion if simply invested in an index fund (according to the article).
https://www.bloomberg.com/opinion/articles/2015-09-03/should... is a good critique of these kinds of calculations in general - they usually assume perfect market timing
the trump wealth transfer started over 60 years ago. over the long run, market timing doesn't matter that much.
(EDIT: and that opinion piece was not coherent; the author mixed up trump's businesses with his net worth, and convoluted other finance concepts to render his desired "opinion". it was awful.)
Besides, there's no claim that he was given anywhere close to $400M over 60 years ago, nor can anyone be realistically expected to invest their inheritance starting at the age of 12.
Don't forget to factor in Trumps' heavy spending over those years which greatly reduces his rate of return. You could just as easily say that he is keeping his value level with respect to inflation and spending the rest.
And at least in the UK people and families that want preserve wealth actually run their own funds. There are a number of listed self managed Investment trusts (with low TER) based on preserving family wealth.
RIT Capital Partners is one example 12.6% pa for 30 years its base was Rothschild family money and there are others Much older.
take trump for example. nytimes detailed about $600+ million in wealth transfer to him from his dad. while we don't have tax returns or financial statements to confirm this, he's probably worth about a billion dollars now. that rate of return is (roughly) less than 2% yearly. he would have been way better off putting that money in an index fund--he'd be worth about $3 billion if he had.