Everything is both difficult and simple. There is a worthy replacement for Bitcoin, this is Ethereum. But the problem is that Bitcoin has a large market capitalization due to the fact that it was the "first". Ethereum will need a considerable time to get around it.
I also believe that everything is not permanent including in the cryptocurrency market but it will take time for a promising altcoin to overtake the leading cryptocurrency. Ethereum must achieve what the bitcoin accomplished for many years to be able to gain more market capitalization and level the playing field. Rather than being defeated, bitcoin will continue the crypto world in so many years before another one will overtake its number one ranking.