Exactly. There's no way Cardano or any other cryptocurrency could replace Ethereum, since Ethereum is extremely popular and actively traded on the market. Since it has first mover advantage just like Bitcoin, it has managed to stay as the second largest cryptocurrency by market cap to date. However, I do believe that Cardano will rise to the point where it could become closer to Ethereum in terms of development and innovation. Right now, Cardano is in its baby steps as it develops into a complete smart contract platform for developers to create dApps on it.
It may take some time before we could see Cardano being useful in crypto land, as its only purpose in the moment is for sending/receiving payments. Once Cardano's VM is up and running, and the ADA cryptocurrency begins the staking phase, expect to see a lot of traction gaining on the Cardano platform as a solid smart contract platform for individuals, businesses, and developers alike. As you said, XRP cannot replace Ethereum, simply because it's centralized and highly manipulated within the cryptocurrency market. In addition, it's not a smart contract platform whereas Ethereum is both a cryptocurrency and a smart contract platform which is decentralized.
Nevertheless, time will tell us what lies with the future of Cardano as a solid contender against Ethereum within the cryptocurrency market. Just my opinion