First. It is necessary to consider what size of deposit. After all, if a person starts trading for example with $ 1000 and he buys bitcoin for all money, he initially puts himself in a bad position. Since we all understand that having bitcoin is good, but it doesn't always make us happy with him price. Therefore, a person will be a hostage to his decision to buy only one coin.
The second. Yes, the price of altcoins depends on the price of bitcoin. But if bitcoin increases by an average of 5% of the cost, at the same time, a lot of altcoins gain 20-30% of the cost. Accordingly, the altcoins have the opportunity to earn much higher. Of course, if the price falls, this pattern plays against the altcoins, so it's important to be on the alert.
In any case, everyone chooses what is more convenient for him to trade with. Personally, I prefer almost always trade with altcoins. But I also remember to accumulate bitcoin.