what parts of flash are not open? When flash opened those parts you mentioned? do their knowledge and control of the closed parts of flash enables them to build better tools? how big a lead did they have for their tools when they opened those parts of flash ? Are they still today the leader in flash tools so maybe it's not worthwhile for a competitor to enter?

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.