There is no law that Bitcoin could not be replaced by some other coin some day. Presently, I do not see a real competitor but who knows which coins will be created in the future. Monero or Ethereum are candidates whereas Eth has a different nature compared to Bitcoin. Upcoming ZCash could also be a candidate. We will see in the future. By now, Bitcoin is king in crypto land 
I do not think that this can even happen, the bitcoin is so big and the altcoins are still pretty small compared with the bitcoin.
Maybe in the far future that this can happen but I do not really believe in that it can happen if I have to be honest.
Maybe in the far future that this can happen but I do not really believe in that it can happen if I have to be honest.