I mean, 21 million isn't much. 
That there are just 21 million units is a common misconception, which we should get rid of.
We can start saying that there are 21000 million mBTC (or mBTC), which is true.
Actually mBTC would be a much more useful unit. As mBTC is currently only about 11$. It would be much easier for new investors to start investing by buying a "WHOLE" mBTC and not worrying if they do not have 11000$ to invest in one full BTC
But yeah. the developers could fork bitcoin and create a new one that has more coins. they probably couldn't change that from bitcoin without everyone agreeing with them.
yup right, the core bitcoin developers can't decide unilaterally about how bitcoin will be developed. Bitcoin has a big name and already has a lot of investors, if they change the supply of bitcoin unilaterally, there will be many people who go from bitcoin. I personally prefer genuine bitcoin where only 21 million is available, no need to be added and reduced.
The reason why bitcoin brings change? why does bitcoin become the king of crypto? yes, it does not escape the amount of supply. The amount of supply has given great value to the development of bitcoin so far.