How will it affect your life if Bitcoin price comes to zero per Bitcoin one day? I have seen most of discussions related to Bitcoin price on pessimistic side only. But we should look at all the possibilities.
It won't happen because with Bitcoin, there is NO MINT function like smart contract altcoins or tokens.
There will be never sudden massive appearance of new circulating supply like Do Kwon did with Terra and LUNA token (now is LUNA Classic LUNC). Without black swan event like this, Bitcoin will hold its price very well.
The trading volume of Bitcoin nowadays is big enough to avoid massive dump from market manipulation like what you are worrying.
How is the 21 million Bitcoin cap defined and enforced?Right. Otherwise we couldn't have a finite limit of 21 million coins, because there would always need to be some minimum reward for generating. In a few decades when the reward gets too small, the transaction fee will become the main compensation for nodes. I'm sure that in 20 years there will either be very large transaction volume or no volume.