I think nothing will replace Bitcoin since bitcoin is more like digital gold than a platform like ethereum, so bitcoin does not have direct competitors. I think in the future we will see a lot of altcoins that are better than Bitcoin with faster transactions and technologies but they will not replace Bitcoin. I think a lot of whales and miners get rich through bitcoin, so they will support bitcoin. In addition, the bitcoin blockchain can be updated to be faster like Bitcoin cash.
I think something completely new will be needed to replace bitcoin, after all even if a coin developed a new way to confirm transactions faster than bitcoin and several other superior characteristics if it is still based on the blockchain, what it is going to stop the developers copy all of those characteristics and apply them to bitcoin? So in a way all the cryptocurrencies are working for bitcoin without even knowing it.