I don't think it's completely a good thing for cryptocurrency if ETF approved btc. There will most likely be a Circuit-breaker mechanism to make the price of btc more stablely. Then it may move the same pace as stocks. Since the quick moving has been attracting people so far, lots of money will go back to stocks. when the price moves stablely, will the money flows form stock market to crypto more than flows away form crypto? that's hard to say