Altcoins have actually surged when Bitcoin goes sideways, this just happened a day ago before there was that quick drop. But you have to realize that they are perhaps surging less than you expect because its the institutions that are driving BTC up. These institutions are only in BTC (and a little bit in ETH), they are not selling Bitcoin to ride some altcoin wave. The retail market will do this, but because institutions are the one driving the market cap up now, there's gonna be a lot less altcoin surges during the times Bitcoin stops surging because the money that is driving the market isn't going to flow to other areas of the crypto market - it's just gonna stay in Bitcoin. Altcoins are becoming more risk and less reward than they used to be as Bitcoin takes a more dominant position as it now is exiting the "crypto-only" market and is entering its phase as a globally accepted asset.