Necessity is the mother of invention and Scarcity is the mother of innovation

The Bitcoin white paper was released right at the peak of the 2008 (exactly 16 years ago) financial crisis when trust in the financial system was lost. The necessity as concluded in the white paper by Satoshi Nakamoto is "We have proposed a system for electronic transactions without relying on trust"

The scarcity of 50 BTC initial block reward isn't defined in the Bitcoin white paper, The 50 BTC initial reward was defined in the original Bitcoin source code.

The 21 million limit is derived from:

1. The initial block reward of 50 BTC
2. The halving interval every 210,000 blocks (roughly every 4 years)
3. The mathematical formula in the source code that controls this

50 + 25 + 12.5 + 6.25 + ... = 21,000,000

The Genesis Block (Block #0) also demonstrates this implementation with its 50 BTC reward. This block was mined on January 3, 2009, and contains the famous Times headline "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks."

Happy Trick or Treat Paper Day!