If we look at the development of the top personal computers, social media like "my space" and software like Microsoft. First to market usually do very well unless they drop the ball or really offend their users or run afoul of regulators (napster) Evenso, they dominate for several years. In the case of bitcoin, there is no overhead, no corporation to go bankrupt etc.
At worst it is like a tool that you set down for a while. You can always pick it up again depending on incentive. The errors of bitcoin core which Roger Ver https://rogerver.com/
has made clear can be corrected and put back on track. Though Ver is seen by some as "controversial" I highly encourage people to look at Ver's videos. He has an excellent understanding of economics and what makes money become money and what make it collapse as it always does in history.
Nobody uses Roman Empire money anymore.
That is for the same reasons that all money collapses ie. printing, printing, printing, making debt and replacing real money with fake money forced on the people by a powerful army. Then the armies stop getting paid or find they can't spend their fake money anymore and it starts to unravel.
This is not a problem for cryptocurrencies if
- They keep the number of coins relative to circulation/use.
- Have a "real world" value or use. ie. salt and grain were once used as money. What real world use does bitcoin have? What about that altcoin I'm looking at? Is it actually contibuting something other than hope and promises.
Will bitcoin be replaced?
To some extent it has been. Look at the history of Litecoin, https://litecoin.org/ Bitcoin cash,https://www.bitcoincash.org/ Monero https://getmonero.org/ and other great coins. Short term they probably took market share from bitcoin. Longterm could we say they are helping to gain widespread attention to this revolutionary NEW ECONOMY?
Cryptocurrencies are not just new products. Building out the blockchains these are built on (the legitimate ones) will build out NEW ECONOMIES.
As for competition: In a small space (relatively speaking) I think it is good and brings more attention. I suspect one day soon many will merge the way stocks in companies do and they could wind up strengthening each other rather than hurting each other.
thanks to 
https://steemit.com/bitcoin/@johnadams/why-is-bitcoin-a-big-deal