It's always possible that another cryptocurrency could overtake bitcoin as #1, but there's only one altcoin that currently exists that could possibly do that and I really don't see it happening. Another cryptocurrency could be invented and possibly overtake bitcoin one day, but bitcoin is currently the most secure, most decentralized, most valuable, most adopted, and most fairly launched cryptocurrency that is currently in existence, and it also has the biggest infrastructure.
When bitcoin was invented by Mr. Nakamoto back in 2009, did he expect that bitcoin will see that much popularity that it is enjoying today. Did Mr. Laszlo think that his pizza purchase would someday be valued at few hundred millions? I guess no!
So I personally think we should not assume that bitcoin will forever remain as a king of the crypto world. It will definitely remain as a pioneer cryptocurrency of course.
It may happen that some other cryptocurrency will take over bitcoin in the long run. Sadly bitcoin is mostly being used as an investment rather than a currency and for a good reason! It is very much possible for people to adopt a faster and cheaper alternative to transact money! You never know!