First of all, anything can happen in crypto world. Right now there is no huge barrier in the way of Bitcoin due to which it is growing at a great pace but future is unpredictable and hence crypto. It is totally possible that people may dump Bitcoin in the future and move to different alt due to some strong reason.
Secondly, if Bitcoin stays long in the market, Ethereum can hardly surpass Bitcoin basically because there is limit on supply of Bitcoin but no limit on Ethereum. Moreover, Bitcoin price is comparatively higher than Ethereum but Ethereum growth rate is higher than Bitcoin. Also, there exists a speculation that if Ethereum growth rate goes the same way it is right now, it can also equalize or overtake Bitcoin in future. Lastly, one should quote Bitcoin may be the king of cryptocurrency but Ethereum progress cannot be ignored.