I think Ripple, Dogecoin or some of these other altcoins have the potential to be the world's currency, but a world currency will need to be politically backed instead of intrinsically backed like Gold, so who knows which one will win out in the end.
And this highness of bitcoin is just due to the yielding percentage of profit, nothing else make a huge difference like this. Altcoins are there but not so much efficient so yeah bitcoin is there for you on first priority.
On the other hand, there are people who adopt other coins too for long term holding also. We cannot be sure all the coins will be having bright future but compared to any other coins, bitcoin must be the only one cryptocurrency which is having highly speculated bright future.