Question about KEVM

I believe if Cardano can do whatever ethereum can (security, functionality) and provide much better performance (much lower fee, much better throughput) then we can replace ethereum as the biggest smart contract platform in the Goguen era. After the release of KEVM devnet, I tested the functionality and security of it. Yes, we can write solidity on KEVM and deploy it. However, I am not sure about its security. As what I've learnt, the KEVM will be plugged to Cardano mainchain with the sidechain technology. Does it mean we need to run a new PoW blockchain? The question is who will maintain this blockchain? Is it not secure, since PoW blockchain like ethereum classic had faced 51 attack several times?