Bitcoin has proven its potential over these past few years and we can't deny the fact that no altcoin can ever replace it.
This question should not even come to anyone's mind. Of course altcoins are always secondary to bitcoin and cannot replace it. Why do some people even come under the pretense that the opposite is true? Because of wishful thinking that one day the shitcoin they hold will become big - but that will not happen.
As for becoming a leading currency, yes, bitcoin has the potential to be one.
This is debatable.
Many people and countries are already accepting and legalizing bitcoin but, it is still not enough for it to be a leading currency all around the world if it is not totally accepted and recognized by all the government. In order for it to be one of the leading currencies, it should be accepted by all and should be totally adopted.
As easy it is to say it, it is that difficult to create such a system. Governments have shown their reluctance to allow the use of bitcoin for most economic systems and the chances of governments allowing its use is very unlikely. Its acceptance is being improved gradually but slowly.