If the supply of Bitcoin is regulated, how could it be said to be decentralized? The minting of new bitcoins is deterministic, and there will eventually be a cap to the production of new bitcoins. Why do we say that bitcoins are a decentralized currency if they are created in such a "top down", regulatory way?
Bitcoin is decentralized because the authenticity of all transactions depends on the 'decentralized' network of miners who compute SHA-256 proof of works hashes. The Bitcoin cap is completely arbitrary and doesn't really matter since Bitcoins are divisible to multiple decimal places. Whether there will be 2,100 bitcoins or 21 trillion bitcoins, the cap doesn't really matter. Its all relative. I feel as though the 21 million number was picked with consideration to the number of potential users in correspondence to the size of ints, floats, etc.

Many people debate as to how 'decentralized' bitcoin really is. It seems as though those who create or own ASIC miners are the ones who really have the edge on the market. Some say that certain altcoins that cannot be mined with ASICs may be more decentralized, since CPUs and GPUs are more widespread.