China banning the mining isn’t decreasing the supply, it’s just slowing down the rate by which the supply (in circulation) increases and hence the rate at which the value of the coin increases

This is only true for the very short term. I'll copy a comment I made yesterday

The amount of coins being made will be more or less the same. Block difficulty is adjusted every 2016 blocks (around 14 days) based on the time it took to mine the last 2016 blocks so that on average it takes the whole network 10 minutes on average to mine a block. New coins are made each time a block is mined.

The only way it reduces the amount of coins being mined is if the processing power dedicated to mining has a sharp drop well before the difficulty update, taking a longer amount of time to update difficulty. Even a 50% drop in processing power right after the difficulty update would only double the time until the next difficulty update (to 28 days), and then the update would make it far easier to mine a block, to bring the average mining time back to 10 min/block