It is hard to say whether any of the blockchains can substitute Ethereum as this blockchain has been enjoying years of widespread adoption. Besides, the Ethereum 2.0 upgrade conducted in the summer of 2021 actually aimed to resolve the problems that both Cardano and Polkadot target. Thus, to take the place of Ethereum, both of these blockchains need to pass an increased demand test and prove their sustainability to attract a more significant following.