I believe that Ethereum will overtake Bitcoin in the near future, since it has really great potential and is also supported by large community including big companies. I think Ethereum is one of the best candidate in the cryptocurrency market for long term investment.
Since January 2018 the etherium depends on the price of bitcoin. Therefore, it is no different from other market tokens. If bitcoin will fall, then the etherium will also depreciate. In my opinion, buying it for investment for a long time is not the best idea.
Please remember that the crypto mark is still too volatile and unpredictable. That's why this is very normal that Ethereum depends on bitcoin price like the other altcoins. But I think this will be changed when the number of the specific use cases of Etherum network grows. I see no reason against this.