Don't say anything finish, remember the only permanent in this world is change, so there is always a possibility that Ethereum can be replaced by any of the existing cryptocurrency or the upcoming one. Even Bitcoin can be replaced by something much more realistic and practical but the only question is when. About Cardano replacing ETH, well if cardano strive hard and beat the ETH in market, development and popularity then it can possibly beat ETH but I think it will take a very long way and ETH need to be stagnant.
One should distinguish the areas in which Cardano might overthrow Ethereum. Thinking of smart contracts and Dapps I would say yes, Cardano has all potential to overthrow Ethereum. Thinking of marketcap I would say NO, it will never happen because of enormous supply of ADA envisaged by Cardano's PoS protocol. Thinking of the mass adoption I would say they will reach equilibrium state, eventually.