I like Cardano and Charles Hoskinson a lot, but I don't think it will destroy Ethereum. Cardano has a better foundation as it's written in Haskell and the developers learned from the mistakes of Ethereum, but Ethereum has a massive ecosystem of developers and partners. Though Ethereum has been slow to implement upgrades, the upgrades eventually come. As the first-mover among smart contract platforms, Ethereum is likely to remain in the top five cryptocurrencies over the next few years.
Yes. The one true thing which has given Ethereum success over many other alternatives available on the market, is its first-mover advantage. No matter how Cardano tries to implement new features, and develop a secure platform for dApps, people will trust Ethereum more for their daily transactions. The situation becomes even worse for Cardano, as its development and innovation are slow. On the other hand, Ethereum is progressing at a fast pace, with a large support of developers and individuals alike.
As for the future of Cardano, it's expected to be a bright one since many well-known developers are working on it. Not to mention, Charles Hoskinson is very knowledgeable on Blockchain technology and Computer Science like Ethereum's Vitalik Buterin. As time goes by, Cardano will be able to grow alongside Ethereum as a solid smart contract platform for the whole world to enjoy. But it'll never destroy Ethereum, in the same way Ethereum won't be able to destroy other alternatives available on the market today.
Nonetheless, it's good to have diversity within this space, as it will allow smart contracts to mature and become widely capable of handling real world situations. As such, Cardano is as good as Ethereum in terms of being a stable smart contract platform for the world. Just my thoughts