You can already produce *.swf - which has been opened for years - as well as reverse engineer *.swf's - which others have done. What is closed up is *.fla files, those are binary. You can code and compile Flash without even buying their software with other alternatives that produce Flash.

Now, as far as a monopoly, they had to basically fight - and won - against VML, SVG, EVA, Silverlight, Java/JavaFX, UIRA (which disappeared). It's had competition; nothing open or competitor was better (pushed) than Flash.

It would be a monopoly in my humble opinion if they simply gobbled up the others or there wasn't any competition. The lack of good competition doesn't denote a monopoly.

When in doubt, look to Microsoft. They bought out their competition instead of beating them. Macromedia/Adobe basically have the Flash Platform that's been upgraded, extended and marketed in such a way that it has stayed a couple of steps ahead in some areas, easier to implement in quite a few areas, has an easy to use/master IDE and then has options if you decide to not use Adobe products if you choose and still produce full-fledged Flash files.

I can't say the same for Silverlight.

Proprietary... yes. Monopoly? I'm not convinced. And opening it up (I'm curious) to accomplish what exactly? I think that's what the Open Screen Project wishes to do - keep Flash in the hands of everybody by actually working with the vendors to keep the experience the same across the board.