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.
mBTC is useless, you might as well use satoshis... The exact amount isn't exactly 21 million plus 8 zeroes, there are some satoshis off due to historical reasons, but not many.
Besides 21 million BTC will be reached by the year 2140.
I only use bitcoin and satoshi, don't need anything else. 1 bitcoin is 100 000 000 satoshis, and that's it. One coin from my country buys one satoshi now; and due to it being the worst hyperinflation currently in the world, soon you will need two coins for 1 satoshi, and so on.