If that happens then bitcoin will no longer be a decentralized cryptocurrency and they will be able to control its price and the volume of the bitcoin in the market will also be regulated. But due to bitcoin's volatility it won't happen in the near future unless its price became stable just like fiat money but it won't happen too because its price depends on the volume regulating in the market. Unless all the bitcoin has been mined, its price will remain volatile because the supply keeps on rising as well as the number of holders.