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.
First and foremost, there's no such thing as forever. We human beings have brilliant minds and every year there are new inventions or projects that are being discovered for the benefit of mankind in the name of technology. We are moving forward into a digital world where people are hungry for new ideas and innovations but I'm pretty sure that the dominance of bitcoin will prevail up to 20 years or more. Bitcoin is the mother of all coins and it's not that easy to replace the very first application of the blockchain.
Every success of a new crypto that's being introduced into the market depends on bitcoin's value. So as a whole if bitcoin's value falls so as the alt coins. The best thing we need to do is not to make bitcoin's image a bad one instead promote it for the benefit of the crypto market as a whole.