For me the answer is obvious. Bitcoin is a more reliable value. This is happening even now. In the long run it will be even more pronounced. The mechanism of inflation underlying the Ethereum will never allow him to become promising in the long term. He's probably already reached my limit.
Demand and supply also affect, that's what happened to bitcoin at the moment. If at some point, the demand on etherum increases but the supply of etherum in the market is less than demand, it is not impossible that price of etherum can also be like bitcoin.