I think that the main token which should Eth holders a bit worried is EOS. if they do what they say they can then it might just destroy the dominance which Ethereum has in the smart contract platform.
Could you briefly explain how they're better than Ethereum, which are the most important points?
each coin will try to compete with other coins to fight for position and beat each other with another coin. as Ethereum is competing with many coins like Ripple, NEO, EOS, and Cardano. indeed these coin coins become coins that compete with each other with Ethereum.