We see that's some of the cryptocurrencies ranking level are decreased such as dogecoin before it was ranked #4 but currently I think it doesn't listed even in top #10 ranking... So can bitcoin also decrease its rank in the future or it will remain #1 rank forever?.
Nothing last forever, the reason why Bitcoin is still number one because it is still and proven to be secured and all the other coins cannot challenge its dominance, as long as this remains, Bitcoin will remain the number one coin in the market, but we cannot say if it's going to last forever.