I view it like how a kingdom works. Bitcoin is the first king, the founder of crypocurrency without him any other coins won't exist. By the years passed by it will grow old, start to suffer some disease high transaction fees, clogged network etc. We do treatment by doing this hard fork but still he can't live forever. With this in mind is Birth of the Prince/Princess (Altcoin) which have more capabilities , much more flexible (higher blocksize), etc. Soon the current king will die and a new leader will be choosen, who it might be? We still don't know.
For me Bitcoin has few more years before it will be replaced as the top coin. It will not die but his current seat will soon be taken by someone.