Ethereum has been published since 2014, since that time it has accompany Bitcoin as crypto currencies coins which has many supporters and market participants interested in investing in it. As the progress of Crypto currencies was rapid, there are more coins created after such as LTC, Ripple, Stellar, etc. I think Ethereum can challenging Bitcoin as soon, but the fact this coin also has many competitors of its own, so in my opinion Ethereum not yet able to pass Bitcoin but still be top of altcoin other.
Ethereum on the other hand is a blockchain smart contract platform (token) which has more use case than bitcoin, but there are also other new blockchain application platforms that are far better (at least in concept so far) than Ethereum. Lisk is one of them. Lisk is scalable as opposed to Ethereum (which is huge) and allows decentralised apps and smart contracts to run on its side chains without exhausting the main node.
I can see Ethereum facing major challenges with the generation of big data and millions of contracts and applications that are going to be executed within the ethereum main network every minute in near future.