Most of the speculators before and even now are saying that if Bitcoin goes up, then altcoins goes up as well and good projects perform well. How true are these speculations if there are different technologies or plans for each cryptocurrency? And the fact that bitcoin is not even the first cryptocurrency.
Actually my understanding in here is that it is reverse proportional. If bitcoin goes up, alts goes down. That's why we have something we called btc dominance in which bitcoin dominates other coins. This is based on my experience during the bull run last last year and then eventually alts goes up as well. When btc became stable.
Typically BTC dominance results in the alt-btc ratio going down, but the alts still increasing in fiat value. However usually that fiat increase is less than whatever btc has increased by. So it's sort of profitable, just not as profitable as holding BTC.
Of course there are exceptions, like 17/18, when alts outpaced BTC. Or when coins do their own rise, such as ETH did years ago.
Over the summer we kind of saw what you referenced though. Alts decreased in both their btc ratio as well as fiat, which was a little odd. Actually alts crashed regardless of which way btc moved, up or down, alts went down in both fiat (not that odd) and their btc ratio (which was a little odd). Perhaps the latter was related to ETH doing poorly, sometimes if ETH does well, alts follow suit.