> Imagine a world in which prices regularly go down

That world results in a lot of people individually deciding "why buy now, when I can buy for less later" and sitting on their money.

That in aggregate makes the economy much worse.

You're up against human nature here. Money may be an arbitrary numerical denomination of value, but people's behavior around it and how that affects the economy at large need to be accounted for. Having prices slowly creep upwards over time (low inflation) tends to result in more, better things sooner.