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?.
It is Bitcoin.
For years and years of volatility, of falling and pumping within weeks or days Bitcoin stayed at the top beside other cryptocurrencies that follow those rankings. It will stay there since it is Bitcoin, it is what it is. The existence of Bitcoin laid the existence of other cryptocurrencies, it is majorly because of the technology but still, Bitcoin will always stay at the top.