and for someone to just artrarily fix the supply of them at 21 million doesn't make any sense.
It's part of the protocol. While you could propose to change the supply and how often it's generated, majority people is unlikely to agree with such change.
it should be able to contract and expand the # of bitcoins in existence.
1. Who gets to decide to change supply of Bitcoin?
2. How would the change of supply works? Increase/reduce value of each UTXO?