I don't think hard forks will ever be a threat. They're improved Bitcoins, to a degree, but being premined hurts them. Other altcoins are more of a threat, but I don't think the one that will eventually topple Bitcoins has been born yet. Ethereum can come close, but it has several issues of its own.
a hard fork that was supposed to be a threat to bitcoin users, is turning into an advantage. for example that happened in august, hard fork that makes bitcoin split in two, this even become the advantage of some parties, so I think only altcoin trigger the threat, because coin that is broke