I do not know how long they will be the king pf cryptos because they are the pioneer so it is hard for them to be replaced because in the mind you the people bitcoins is their first thing to be known and the trust and the faith in in the bitcoins not in the other coins especially altcoins a lot of people are thinking that altcpins are just a shitcoins and scams. But as we can see there are a lot of altcoins that are rising up and building milestones so maybe bitcoins will be surpass by another coins we cannot know we cannot predict the future just chrish the moment.