I don't think it leads to a decrease in economic activity, just discourages an increase.
Why doesn't it lead to a decrease? Well, as soon as the economic output decreases (but the money supply didn't change!), then each dollar is worth less, so the incentive (to not spend because it will be worth more tomorrow) disappears, so the spending starts back up...
Why doesn't it lead to a decrease? Well, as soon as the economic output decreases (but the money supply didn't change!), then each dollar is worth less, so the incentive (to not spend because it will be worth more tomorrow) disappears, so the spending starts back up...