That's a common myth. The halving does not decrease the supply. The supply is always increasing until it reaches 21 million. You assume that the demand will continue to increase, but that is not guaranteed.
It is not a myth, it is just you nitpicking
Or rather deliberately misinterpreting the meaning of the term in the given context. Obviously, people mean the supply of new coins to the market which is evidently diminishing after the halving, as it is meant by the law of supply and demand (I wouldn't call that law a myth either), which people implicitly refer to here. What you mean is total supply of coins in circulation, but the distinction can be easily drawn from the context, and in this case specifically your comment is totally out of place and beside the point
You are correct to a certain extent. I am, but only because I'm too lazy to post a full explanation every time this misconception is stated as if it were a fact.
TL;DR: The halving does not reduce the supply. Instead, it reduces the influence by miners (as a group) on the market.
First some distinctions are important. There are two meanings of the word "supply".
There is the
money supply. It is the total number of available coins. It is a number, though its value might vary depending on what is meant by "available".
Then there is the
market supply. That is what is meant by "supply and demand". It is a curve. One axis is is price and the other is the number of coins that can be bought at that price.
All coins in the
money supply, including "new"coins, are represented somewhere in the
market supply.
people mean the supply of new coins to the market
First, there is no difference between "new" coins and "old" coins. When people write about "new" coins and they call it
supply, they really mean
production. Those are two fundamentally different concepts. There is no such thing as "production and demand".
Second, stating that the halving reduces the supply implies that the number of coins in the market is now less than it used to be because the subsidy is 1/4 of what it used to be, and it implies that when the subsidy falls to 0, there will be no more coins for sale.
Of course, that is nonsense. Coins are not consumed. The coins in the market consist of "new" coins plus "old" coins, and the number of "old" coins available for sale is constantly increasing.
So in summary, when people write that "the halving reduces the supply", they are wrong on a few counts. The money supply is always increasing, and the market supply is always shifting to the right. Neither supply is ever reduced by the halving. Instead, the halving reduces the number of coins held by miners, so it reduces the effect by miners (as a group) on the market.