I don't think it will kill Ethereum, it just becomes another possibility to run transactions on.
It will become a competitor but it will not kill Ethereum.
It will be another alternative chain for ethereum and that's the only thing that can be achieved by DOT. It's quite impossible for DOT to replace ethereum as the best smartcontract platform.
People are putting ethereum as the best choice for the tokenization purpose and defi development.
They will not try to move into the another platform.
Some projects that decided to build their dapps on DOT just wanna to get the ride for the hype.
You are right that it will not be easy for Polkadot to replace Ethereum because a Huge number of Alt coins have created their smart contracts on Ethereum network that is why there is congestion and we have to pay huge Gas and Transaction fee. Scalability is another issue which Ethereum IT team is trying to resolve but so far no significant improvement has been witnessed. Polkadot works works on Multiple chain concept that is why transactions are executed fast on its network. Polkadot dot has made huge progress during past one year Now it stand on Rank # 4 on coinmatketcap which is great achievement.