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But still, it's more efficient to have a central planner, and to have a dynamic supply rate. If supply rate isn't regulated to cause price stability, then it would cause you a lot of problems if you are in finance or even if you're an entrepreneur. When doing prognosis, then you have to calculate in the possible inflation (or deflation for that matter). When the inflation rate is stable, then you can create more solid prognosis and predict the needed numbers. With fixed supply and unstable value of money, you can never be sure which way will it go next year, or even next month. Doing business will be more of an lottery, and that's not a good thing. It's not good when competition is decided by luck, not by competence.
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Here you propose that the inflation rate (of prices) can be stable. But if it is, it is just a matter of computation to convert from a zero inflation rate to some positive inflation. To suggest that people can be fooled into spending, or workers fooled into believing that the value of their wages don't go down when they are nominally fixed, is foolish, and an expression of a master mentality.
Even if it is theoretically possible to have a stable inflation rate (in prices) or a zero inflation rate, it is not possible in practice, because 1. the point of the whole thing is making it unpredictable, to shake out those calculating with a stable inflation, 2. it is counter to the interest of the elite, 3. there is always a crisis that needs to be curtailed with printing and 4. even with the best intentions, it is difficult, after an expansion, to tax more only to rein in the extra fiat.
A zero inflation rate (in money supply) does not necessarily imply unstable prices. The typical seasonal demand variations for money can be countered with temporary expansion in private credit level between firms in the different production stages. A general increase in the savings rate
should lead to price reductions (including interest), also increased productivity
should lead to price reductions. A natural disaster destroying vast volumes of capital and consumer goods,
should lead to a price increase.
An increase or reduction of population will effect prices, unessesarily maybe, but it doesn't matter, because it is known to everybody and they can adjust.