When the price of bitcoin goes up a lot, altcoins will drop in the short term as people either sell to ride on bitcoin or convert to fiat for cashing out.
As the bitcoin bull market continues, people will gradually turn to alts as the price of bitcoin looks expensive compared to other coins. Then you will see huge rallies in altcoins as everyone rushes to find the next bitcoin.
Most altcoins do really have decrease on their price when bitcoin price increases and you are right on your points they are riding the bullish trend but there are some alts doesn't tend to decrease its price infact they are sticking to move up.I don't know whats the true reason behind this since im not too expert on crypto trading.This thing is just what I observed.
Now, it is better to buy bitcoin.