I mean, 21 million isn't much. 
Being limited is always good, and that's what i think of bitcoin. As a limited edition coins.7.7 billion people in the world and 21 million bitcoin. You do the maths. That would be 0.002 btc each not counting whales and lost bitcoins, so probably a lot less than that. That only gives you a few decimal places to play with. If you actually want people to use bitcoin, then you need to make sure there is enough to go round.
its eight decimal places now. that can easily be extended to 16 (or whatever) or more decimal places as needed. so, no need for more than 21 million. just add more decimal places.
8 decial places is the smallest amount you can use in a transaction. So going to 16 dp would require code changes. You are just making more satoshis as opposed to more bitcoins.