But every day we have new announcement and new projects that can be direct rivals to btc, this is something far more dangerous then fiat.. Bitcoin was first, but we don't use first computers and cars no more.
You have to note that we are still using first computers but in the advanced form, in that case we cannot compare the story of computer with crypto world. bitcoin has its own place and people will give it its value if bitcoin continued to adopt advanced technologies, and if it do not remain as a fossil, and all other coins has its own lace and have their own uses, people are using both bitcoin and a number of altcoins also.