These alt coins are hoping to be the next Bitcoin if sometime it will just die then they are there to take its place which in fact Bitcoin is irreplaceable. Alt coins is a good one to earn profits from it thru trading and some good little things more.
I don't believe in this, everything can be replaced especially when we are talking about software and hardware..Your statement is about what is more convenient for us to believe, not necessary logical and truthful, no one knows what it going to happened.
Already we have examples where few good alts have raising success, this do not necessary means Bitcoin demise, this is what is good part in all this.