An altcoin can replace, that's possible. Only if it will be accepted by most in this community and other industry accepting payments, its okay, no big deal.

Now if you're talking about some existing altcoin that will replace bitcoin, no there's no chance even ethereum or any in the top 10 in cmc. I'm talking here about the hashrate, every alt besides bitcoin, which cannot beat bitcoin in terms of hashrates as for the moment and transactions except eth since it has lots of shit coin integrating on its blockchain.