When you really look at it, there's no reason that Adobe shouldn't embrace HTML 5. Fundamentally, maintaining a cross-platform plug-in is not a profit center for them, it's a cost. They don't make money on the plug-ins, they make money on the Creative Suite product which allows designers to create animations, games, and the like easily. All this work of maintaining their own actionscript standards and standard library just serves to make their pay products more useful.
Imagine for a moment that at some time in the near future, Adobe has a new option on the menu "Export to HTML5". Would this make their product less useful? Of course not. Widespread adoption of HTML 5 means that their product can now be used to create content for even more devices, including several, like the iPhone, from which they have previously been locked out. And it wouldn't even be surprising if over time they transitioned entirely to HTML 5, giving up the work involved in maintaining Flash. They probably won't do this in the short run, but in the long run, it's entirely plausible.
I'm sure some people will point out that the move to HTML 5 opens them up to more competitors, and it does. But they've already got competitors even with the Flash ecosystem. There are a variety of ways to make swfs, including swftools, FlashDevelop, and the free Actionscript compiler which Adobe itself released as part of the Flex SDK. There are even a few other pay products out there. So, essentially, they already are in a market where there are a bunch of other tools which are cheaper but either can't produce complex content or require a bunch of coding to produce similar content. If they switch over to HTML5, they will likely be in the same boat, just in a bigger lake. Sure they'll be competing with DreamWeaver or whoever, but they'll have a clear and immediate advantage when it comes to "Flash-like" stuff such as animations and games.
So in summary, if they manage the transition properly, moving towards HTML5 means less costs and a bigger market. That sounds to me like a pretty clear win.