Bitcoin's current market cap is above 66% of that for all the other altcoins combined. Even if it went down to, say, 40%, BTC still would remain dominant in regards to market capitalization. Bitcoin is the most trusted coin among crypto currencies, and, in fact, all the other alts owe BTC that they are getting any adoption at all. Can any of them replace the king? Maybe, but not in this universe, where, apart from being number one already, Bitcoin has the best team of developers around.
Let's accept the fact that there will be the time that bitcoin will get replaced by a better cryptocurrency. It might be already existing or soon to be released by its developers. Bitcoin is like Nokia which is the pioneer of the cellphone and now it is replaced by the better ones like apple, samsung, huawei and many more. It is not about being the first or being the king, it is about the features and capabilities that you have an advantage on compare to others.
We cannot deny that bitcoin can be replaced but it is never going to be replaced by a coin that is basically using the same technology that bitcoin does as the majority of the coins in the market do, even the ones that are successful, besides you are thinking of bitcoin as if it is a smart phone while bitcoin is more similar to gold, and gold has been around for thousands of years and some economists are asking that we implement the gold standard again so that should give you an idea of how long bitcoin could last.