bitcoin will exist as long as it is needed, as long as it is valuable, as long as somebody use and accept bitcoin and for that we can say that bitcoin will last long as we exist, we use bitcoin in our life and we will make it last till for the next generations.
but if we will not use our bitcoins and it's price continuously fall down then there is a possibility that it will be replaced by other cryptocurrencies that is more convenient to use but we on't see that coming soon, maybe it will happen but it will takes a very long time to overcome all bitcoin success.