If the fork happened many people will start to think to replace bitcoin place, Ether is not really suitable for replacing bitcoin because ether itself got a lot of problem and been forked before so most of the people not really trust eth, but once it is replace by other alt coin bitcoin will be really difficult to become the main crypto currencies, at that time we will labeled bitcoin as an alt coin
Altcoins people will think about it, but that will never be possible. The main candidates are all corrupted, broken, involved in shady business or crappy. Bitcoin really stand out of the crowd.