We're just that amazing. ;]
In reality what you are saying is that if X happens, you waste alot of time, if !X happens then there is still no guaranteed outcome.
I don't agree that the odds are fixed, nor that it's all-or-nothing-to-the-end. For any startup, you take mortal risk as infrequently as you can; you hedge those bets as much as you can; and, since you can invest effort to change the game in reaction to the market, you work to improve your odds over time.
So in the end, I believe you present a false dichotomy. It's rarely $100MM or death, and there's never a guaranteed outcome.
1) Flash is prevalent today. No matter how much people are bashing it on HN, its widely used especially on the sites which are incredibly popular. 2) HTML5 adoption will be slow. Look at how long it to ie6 to die and you'll have an idea of how long it will take html5 to become prevalent. 3) You're taking a technology that allows you to be accessible today and trading it for a technology which will allow you to be accessible at some unknown time in the future, basically removing any barrier to entry for your competitors.
The hyperbole makes this seem worse then it is, I'm sure. You don't really have motivation to ditch your current flash implementation. You'll just put it into a "support as needed" mode.
Come on, it's great that they're ditching Flash, but if it really works on IE6 calling it "HTML5" is nothing but link/pageview-bait. The parts of HTML5 that are supported by IE6 are called HTML4 and Javascript.