I'm sure, if there is no altcoin then like a man without a woman, bitcoin will die and will not get people interested. Remember, one advantage is the large number of bitcoin altcoin, but I'm sure that one day there will be a coin which replaced the bitcoin.
Agreed, in my view fiat is the man and Bitcoin seems to be the women, and just because of these two several altcoins got into existence. As generations continue one might get overtaken by someone, so possibilities of bitcoin getting replaced by some other coin is possible in the long run.