Altcoin are like Bitcoin because their concept is similar to Bitcoin though they might have different specifications. Now to address your concern. Altcoin is not dollar economy but individual coin economy. The reason many time altcoin is traded against bitcoin is simply because individual coin are not yet spread and saturated yet. And since bitcoin is, they benefit from success of bitcoin. When it come to issue of price you will find out that the price behaviour of individual coin are different. So no matter the price of bitcoin, you will buy each individual coin according to their value in the market and that should not affect your profit or loss regardless of bitcoin prices.
That's the right explanation to the question. Also, to add to your answer I would say that most of the exchanges prefer to trade on pairs with crypto to crypto which makes it easier for them to convert the currencies and maintain the charges. Since BTC is the king of cryptos, altcoins are traded with BTC pairs and then BTC is traded with USD.