It's not only the reduction of the new supply.
it is also:

1. increasing costs for the miners as they get only half the blockrewards, but the amount of energy and hashpower stays the same. (they want to protect their investments so they BUY, the price need to double (13k) for them to not lose money)
2. increasing interest in BTC by Millenials and institutional investors.
3. between 1 and 3 million BTC is already lost forever which shortens the supply even more. So 19 Million BTC total vs. 800 Million Millenials.... go figure.

This is a self-fulfilling prophecy.