Adobe Doesn't Have to Die
This doesn't have to kill Adobe. After all, the Flash player plug-in is given away by Adobe free of charge. The thing that Adobe sells, and makes money from, is the authoring software.
So, if it becomes possible for HTML 5 to permit content to be displayed in a manner comparable to that of Flash, the Adobe software can presumably be modified to save content in that form. Of course they will lose the advantage that fewer people have the Silverlight player installed than the Flash player - but Flash became dominant because of Adobe's superiority in the whole technological field.