Bitcoin currently suffers from a serious scalability issue with no solution on the horizon. Ethereum constantly gains greater acceptance and hosts an increasing number of projects on its blockchain. When will the inevitable occur? My opinion is that in about one year the marketcap of Ethereum will be greater than that of Bitcoin.
Never, Ethereum lost some of its value to Bitcoin. I don't think that Ethereum can overtake Bitcoin, just because Bitcoin is the leader and is the most popular and wanted coin. Bitcoin is running this market, not Ethereum.