For me the only real Ethereum "killer" is ethereum itself. If they are able to finally make the full switch to PoS and if this then means that the transaction costs are going down significantly then i don't think that any other project can really "kill" ethereum. By significantly lower transaction costs i am talking about below 1 USD and perfect would be a fee of around 0,10 USD because then Ethereum would be fully usable also for small purchases and small trades again. If they can do that within the next 1 or 2 years then Ethereum will definitely stay the number 1 smart contract platform and deservedly so, as they have invented all that stuff.