We can not say that bitcoin will be king forever. Say due to the fact that he was the first. But he also has some problems that need to be solved. Perhaps some coin can replace bitcoin, but this will not happen in the near future.
We cannot say because its a technology whereas we know that technology keeps on growing and it will be placed by a better tech. But I don't think that it will happen soonest that it will ever be placed by another coin. There's no other coin better than bitcoin, the replacement can just happen if there will be a coin that will have a better market cap than the king. We believe bitcoin will remain as the king for many years, 5 more decades or more than that.
Technology can be replaced by another technology but i think coin will be the same Bitcoin, because it is not controlled by anyone and bitcoin is the only truly decentralised coin, which itself is served as digital currency usage