Eventually in the future, bitcoin will be replaced by another crypto currency but not soon. Bitcoin will be available till 2023 for mining. After that bitcoin will remain the costliest crypto currency. However, those who have already set up their expensive mining system, what will they do?
They will obviously find another crypto currency to mine and that crypto currency will become the next big thing after bitcoin. Everything in life, has a expiry date and bitcoin is no exception. But that is not going to happen anytime soon.
You will have to explain some details. Why mining will only be available until 2023? Because we should still have a good 100 years until no new blocks are mined. Also the fees will take over as the reward. Altcoins use different algorithms, with ASCIs it is not possible to switch to the next best coin.
Probably people will not look for another system anymore, they trust bitcoin so much that is why the price that we see now is rising. That is the statement of the majority and create some good demand to make it happen. If there is a specific altcoin that will replace bitcoin, we cannot ascertain what it is due to their huge number.