Well, if bitcoin fades into insignificance, it is as good as death.
Given the potential that Bitcoin has, I would say that any alt overtaking it in terms of market cap would spell the end.
No way an alt coin would take over BTC but it could just die in price when it isn't useful to the community anymore, that's the day when we really just get tired of btc and dont need the coin for any reason.
I dont think BTC will die out for a long period of time.
Bitcoin can never die till it holds a great value people will continue to use it, may be price can go down in future but it will always shows its presence.