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.
In this area everything might happen, I don't surprise if ETH will give its place to another coin. Speaking about this situation, I think that ETH is completely fully entrenched in its place and will not give up to someone.
Ethereum has already given way to second place Ripple on the amount of capitalization. But this does not mean that Ethereum can still overtake someone.