This is exactly right - when BTC goes up and alts go down that is the best possible time to buy alts. BTC is the way that most of the money comes into the system, and the traders sell alts and into BTC before that happens causing alts to go down. Then after the money comes into BTC and the price is much higher, they trade back into alts causing a BTC consolidation and a rise in alts.

I have generally been selling over the course of 2018, but now I am a buyer of STEEM (and probably some other alts too)!