BTC isn't going to go away thats for sure. its the only coin that has access to the fiat world... its just that for people wanting to work on a different system with more features, its nxt as of the moment. and possibly ehtereum once that gets released.
BTC will always be king.
Bitcoin's been going strong for years and never really been hacked. No alt has proved itself over the same length of time. People won't trust alts enough to let them take over from bitcoin unless they can prove they are unhackable over a period of five years. Bitcon will remain king for the foreseeable future.