Some of your point sounds a bit like this (please correct me if I'm wrong): "Deflation is always bad because it could be hyperdeflation." Namely, if deflation is significant enough that it incentivizes me foregoing a purchase for a few months (and then another few months, etc) it's _significant_ deflation. But why not make the equivalent argument "inflation must be bad because we can imagine hyperinflation" as well?
But instead of hyperdeflation, if we had -0.1% risk-free interest rates, would people really forego all purchases? I don't see why. If I need a new toaster, or some clothes, or gas for my car, I would not delay the purchase for 6 months to save 0.05% on the purchase. This seems like a realistic number if the deflation is caused by a population contraction in a country with an advanced economy.
It seems real-world encounters with deflation (at least since the inception of central banking and planned expansions of money supplies) have come largely during periods of _sharp_ economic contraction. But is such a scenario really comparable to a slow decrease in population?
Moreover, if monetary policy can offset deflation, what creates the need for immigration to offset the deflation? Why isn't the monetary policy enough?