I cannot be actually sure about the altcoins whether they can overthrown bitcoin one day or not. But, obviously there are many prospective coins in market right now.  These coins are competing with Bitcoin massively in the market. Ethereum, Litecoin, ETC and Bitcoin Cash are all at the for front. And Yes, you have to choose the  one with the potential of growing exponentionally.

Moreover, It is definitely better to buy other altcoins with ETH and BTC, by doing this you will increase your BTC & ETH. Now the thing is altcoins are volatile so you need to be careful and have a strong risk management.

But ultimately by trading altcoins you will get profit in 2 ways
Your BTC will go up in quantity
BTC price will go up in Fiat currency