As someone who got into game development by making Flash games as a kid, I would love to see Flash open sourced. I don't think it's necessarily true to say they have nothing to lose by open-sourcing it though. Who knows how many private shared libraries are in there that are still required by other still-active Adobe software. And they're also probably not excited to give up rights to a massive pile of code which they could conceivably want to use in future projects.

In other words Flash likely isn't some isolated directory they can just zip and share to the world, and even if it is they might want to pick the bones later so why throw it away? (from their perspective)