At the 4H-timeframe scale it can be clearly seen how perfectly correlated are Bitcoin and Ethereum price actions in the middle term.
There are vertical flags with dates and time from the beginning of the observation till the current moment. Starting from the 21th of November you can see each sharp movement duplicates, even IHS patterns were formed in the same manner.
What does it mean? 1. Bitcoin's capitalization and Ethereum capitalization is represented mostly by the same big players now. 2. Once fundamental news arrive and affect the market big players either fix their crypto to stable coins in case they expect drop or enter the market back diversifying their holdings between ETH and BTC in equal proportions. 3. It does not seem like a collusion it is more likely big players want to diversify, however this now destroys opportunities of diversification. 4. Because ETH and BTC can no longer be used for diversification soon we will see whales choosing other instruments for it. 5. Once new coin is determined ETH will have an inverse correlation with Bitcoin. Moreover Ethereum will face with a sharp value drop once it happens.
Conclusion: Do not use Ethereum for diversification in the mid or long term. Ethereum is now risky for long term holdings, choose several top-10 cryptocurrencies instruments for it