Two main points I see lacking in your analysis:
1. The deflationary spiral is presented as some "fact". There is no evidence that this is what happens if there is a fixed supply. It's absolutely logical that people WILL spend less if they know the money will be worth more next year. But how much less? Is that a bad thing? In many ways, all the recent economical troubles have been caused by exactly people spending too much, spending the money they don't have. Maybe deflationary supply is exactly what is needed for people to buy what they need, not go on buy-all-sprees just because they know their money won't be worth anything soon (as it is now in the inflationary model).
2. World history regarding the price of gold. As an example from Creature From Jekyll Island (a book), in ancient Rome, to buy a custom tailor-made suit and a pair of custom-made shoes cost 1 ounce of gold. This is approximately the same order of magnitute of what it costs today, by today's gold prices.
All while Gold is, by many standards, is deflationary as well. (Well you might say that new gold bricks are mined, but it's very limited. In the same way new bitcoins are also still mined and will be so for hundreds of years.) So what happened? If fixed supply is such a bad thing, why don't people just hoard gold and the price is through the roof?
On the contrary, the history shows many examples where after societies went from the gold standard to the more "fluid" money supply, when governments started to mix other metals in the coins, when they removed gold at all - that's when the economic troubles started. Human-controlled money supply has proven itself time and time again non-functional. The people in positions of this power cannot avoid abusing this power.
There are a lot of things that point to the notion that it is in fact the "deflationary spiral scare" that is a joke, and a fixed supply is what is needed in the long run to make money work most effectively.