They has the same purpose, but by different system
If one day bitcoin fade, that will not bring effect to another altcoin but if that happen, there will be big price deduction to the altcoin but altcoin still exists.
The thing to be a problem is if blockchain technology lost then all will be lost, but in my opinion that will not happen because this technology can attract internet peoples to join.
So bitcoin will not ever lost because bitcoin called as the king of all coin