So instead of adobe being the monopolistic developer of tools for the successful flash platform , adobe should compete with everybody else on developing tools to html5? This doesn't seem like a great business move, at least not before they tried everything they can to keep flash important.
But isn't ActionScript completely open? And the flash framework is open too. You don't need Flash CS4 to use flash. You can also program it with a third party IDE and then use the free compiler. This would be quite comparable.
I really don't know flash in depth , but my guess is that they today have a big advantage in flash tools and moving to html5 would seriously reduce their advantage.