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?.
Of course any currency can be replace easily especially if the currency in the bottom will likely to skyrocket the demand so we can say that other currencies have a chance to replace bitcoin someday as number 1 in the ranking if they will be doing their best. But if the investors will always favor bitcoin over other currencies the same to what is happening right now then we can say that bitcoin will always be at number 1 and it will last forever.
So in short, all will always depend to the investors especially the big investors out there if what currencies they like to support in the future because they can easily pump any currencies that they like if they want to.