I seriously disagree, look at the growth of wealth inequality since 2009, "the rich are getting richer and the poor are getting poorer".
If they "join bitcoin" I have no doubt they will pump/dump/manipulate the price to a point that they can accumulate enough bitcoin that they have total control over it. Then it would be like any other centralized/elite controlled currency.
Unlikely, they cannot get rich from pump & dump, the elite got rich from comissions rather than trading.
All major banks are middleman, and hardly trade anything, that is to protect their wealth.
Plus their main income source is derived from printed money, bitcoin is a closed system.
That being said, by joinning bitcoin, they will pump up the price so much, that it will become a real safe haven, and they will have no choice but to stick with it, when the financial collapse comes.
Bonds, stocks, estate, all of it will deflate in price, that is not a good way to make money, especially if the bank or broker you are with will just go bankrupt. In a systematic failure you cannot trade the classical way.
Most rich people just buy up cheap land in poor countries to look for a safer haven, so they are desperate to save their wealth.
Bitcoin would be the perfect safe haven for them, and they will eventually start using it.