If you will compare bitcoin with altcoins you can tell that there is a big difference and with the current situation you can tell that bitcoin are more dominant as most of the legitimate altcoins are dropping its price while bitcoin is soaring.
may be in future but in my opinion it is vety hard to say that any new online digital currency may overcome the bitcoin but still there are many problems because who will invest much amount of money in new coin or online currency .
Its very hard to say its replaceable because its really pioneer and now no one can replace but we can say some can give support like we have in metals Gold is Pioneer and others giving him good support