All altcoins even eth are still depending on bitcoin because bitcoin is overvalued so far.
In order to obtain altcoins,you must go first for the primary digital currency.
Yup, I agree with this answer over the others. Bitcoin is what you can consider the mother hub of the altcoins. A lot of altcoins is contingent on you having some Bitcoin or Eth on you. Furthermore, I think when trading altcoins, people want to stick with cryptocurrency so when they give up some of their altcoins, they can get a different, more prominent coin back as payment in bitcoin.
Also btc and etheruem have there own blockchains so you can trade and coins through there. And most sites would make btc or eth as the main currency so buying btc or ethereum can make you acquire any altcoin you want as much as possible. There are crowdsales who sell there own coin but off course it is risky as they say.