I actually have a billionare mogul. But mine his wealth all comes from his family. He is half Chinese half Korean. But his Chinese father abandoned him and he used his father's money to make some good investments. His mother was from a super rich family and that is actually why his parents got married, it was a business merger, arranged marriage that leads to love. But yeah, I totally get the whole billionare nowadays trend, but they don't have money from anywhere specific or that makes sense to the market that we live in now.
Personally, I think it is okay to write rich characters as long as you do some research on how rich people became rich in the place your character's setting takes place.
Like make your character's back story make sense to the setting they are in. Don't just give them money because... I can.
Case and point. China one of the highest percentages of Billionare's in the world, many of them rich because of real estate. My characters father real Estate Billionaremother Advertising giant conglomerate worth Millions of Dollars. And there are parts where I actually show them working on adverts. XD