Like all digital technology, it is unquestionable that sooner or later bitcoin will have to be replaced by a superior technology, just as in the past we saw many devices become obsolete when at the time seemed insurmountable, such as CDs or PCs.
Obviously this will happen in a gradual way, but I do not believe that bitcoin can retain its current dominance for more than ten years before starting to fall into disuse, although perhaps its influence could persist in the market about ten years.
In the end, bitcoin is likely to subsist for many years, but by then it will have evolved in such a way that it will be virtually unrecognizable for todays standards, and in fact it is difficult that in 15 years there is something similar to the current cryptocurrencies before the spectacular advances in digital technology that are approaching.
Let we say that there has a possibilty that bitcoin will have a competitor even now there is Eth and other coins that want to become number 1 in ranking. As of now, bitcoin is in the top rank lets enjoy it before it will loose its value because of coming coins that will won against bitcoin. But as of now, that bitcoin is popular to used it will not defeated. Lets believe to want bitcoin will do and continue to explore to it.