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.