If the government and banks hold 90% of bitcoin then the volatility of bitcoin will become static and the value will not rise for a long time. The market forces of demand and supply determine that price of bitcoin. The government will have to spend billions or trillions of dollars to buy that amount of bitcoin so its cheaper for them to ban than to undergo such an expensive venture
Governments and banks fortunately do not hold 90% or even 50% of the crypto currencies. If you say that they hold 90% of the bitcoins, they would have stopped ordinary people from using it and keep the banking system in place forever.
Why would they? Btc is a great tool to rip off people that don't pay taxes and also rip off hidden money. So btc is quite a smart move if designed by authorities. The original purpose of btc is bright, but what do we have now?