i think the internet will never die unless we have a full out world wide disaster, in which case, we have bigger issues.
the internet is to big, and if one part fails, there are a ton of other parts to it that will still be up.
i also dont think there will ever be some thing else to take its place, the internet is a network of computers. we may have different ways of connecting all of them, but in the end its all the same.
the internet as we know it might change, after all in my life time it has changed, i was online a year before Google started. and we all know Google has done a lot to change the internet.