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.
How true it's can be observed from previous price movement of the market, altcoins tend to follow-up the full steps of bitcoin as it dictate the movement of the market. You might want to argue with the current observation from the market as bitcoin recently tends to noon without carrying the altcoins with it but if you pay close attention you'll observe few selective altcoins are adding price value each time bitcoin price increase (Litecoin is a perfect example).
The phrase doesn't have to reflect all the altcoin everytime but it's a ptoven fact that bitcoin controls the market most especial as altcoins are traded with BTC pairs and it's dominating the market. This is an emotion driven market so a decline in price of the most trusted asset has to reflect on the inferiors. Bitcoin is also the first, decentralized cryptocurrency others before it were centralized digital currency.