It's a good question, and if you look closer you'll see that ERC20 price movement is more in line with ETH than BTC.
But I don't think ETH and BTC are truly detached so it doesn't impact analysis all that much.
The price of altcoins is still dependent on the price of bitcoin. This will occur as long as Bitcoin will maintain a high level of its capitalization. However, the level of Bitcoin capitalization tends to a steady decline. The day will come when its level of capitalization will fall so much that it will cease to dominate in price terms over other types of cryptocurrency.
It seems to me that this will be the best time for the development of cryptocurrency.