Cardano is a good project, but I don't think it can replace ETH. ETH is has more utility, more DApps, solid and experienced team on the verge of making ETH more faster thus more scalable.
ETH - head start, great projects, huge community, novel (buggy) programming language, but it's also one that most people have already learned.
Cardano - weird programming language that no-one outside of universities likes to use. They should have at least done like Tezos and picked a functional language that at least has some use in the real world