What will replace Bitcoin? Or what will Bitcoin morph into?

The last bitcoin will be mined around the year 2140. I know that the coin will still have value due to network transaction fees and that it will still be fully tradable, have value, etc, etc. Will transaction fees be high enough to make it worthwhile for miners to still verify transactions? If transaction fees rise, will there be less transactions?

Something has to come along to either "fill the void" or be the next big thing, right...right!?!? So what is it? Does it already exist? Humans like the next big thing, so something will come along that will be the next advancement in storing value/bartering/exchanging, right?

If your answer is "nothing, bitcoin is all we need" -- how the hell do you know that? It seems completely naive to say something like that. I guess I'm asking, where does this go in the next 100+ years? It can't just stay exactly how it is right now can it? What do you see changing?