So bitcoin will disappear soon? What do you mean of lets just accept the current bitcoin as it is? If there is only 21 million and that is really it is, is that mean bitcoin will going to be zero in the future? Maybe bitcoin supply will also increase especially if therr are also a lot of us will mine.
The number of people mining is wholly irrelevant. If one billion people were mining, there would still only ever be a maximum of 21 million BTC available. that's a hard limit. Miners are irrelevant.
The developer can do such thing without information so how i can we notice that bitcoin supply will never be increasing? I have no idea about this.
However, if the supply increase, it will make the trust to bitcoin loss forever.
If a developer were to increase supply, he would break consensus and blocks generated by such a piece of software would be rejected by anybody opposed to it--which I assure you will be the majority of Bitcoin users. Developers can change the software but they can't force everybody to switch to that new version.