Polkadot is currently the toughest competitor for ethereum compared to other platform blockchains like eos, stellar, waves, neo and it is actually very difficult for polkadot to become ethereum killer considering ethereum still has advantages over this, but if ethereum makes a mistake it's not impossible polkadot will become ethereum killer.
Ethereum has face tougher competitions in the past just amongst the projects that you have mentioned. And so far, there are not even close to what Ethereum has achieved, so I doubt that Polkadot will be this Ethereum killer because there is none. What we are seeing is that Polkadot is just being hype, specially that we are in the bull run already. Every new project will have this kind of hype, similar to 2017. But I doubt that they can get even close to Ethereum as far as support or consensus in the altcoin community.