Adobe's only real hope is to open-source and freely license the whole Flash stack. They've proven repeatedly that they can't manage the evenness, performance, and ubiquity necessary for something like Flash.
If they don't do this, Flash is dead, which is stupid imo because they could still make a lot of money off of the IDE.
There's a benefit from a publisher's view point to focusing on a single standard.
If HTML5 gets similar market share to Flash, I imagine in the long run most content publisher will choose the one that's better supported and easier to implement rather than maintain support for both - for the same reason that many now choose only Flash, rather than Flash+Silverlight+whatever.
If Flash opens, HTML5 adoption won't matter as much. It would still be a nice goal, but its urgency would be greatly diminished.
Flash already meets a large portion of the needs addressed by HTML5 but Adobe's blobs are almost universally inadequate. HTML5 immediately becomes less attractive in the face of an open Flash, as Flash has a huge library of existing content, a huge base of existing developers, and Flash does much of what HTML5 does, but Flash is ill-favored right now because Adobe holds it far too close to chest.
An open Flash would mean implementation on the iPhone and other important platforms.
Some might say it's too late for that, but I don't agree. Flash is on the brink right now; if it opens, it can have a place forever. If it stays closed, it will die away forever in favor of HTML5.
What if Adobe acted more like an ActionScript standardization committee than the exclusive Flash vendor? Leave the implementation to groups who can customize it according to their needs (like, maybe they need Flash to work on a capacitive multi-touch device, or maybe they need it to work well on a non-Windows OS or non-x86 platform).
Flash wants to play as an integral part of the web, but it will never be able to last if it doesn't get free. The internet was able to grow to what it is today because it is built on free techs.
Flash is at the point where it has outgrown single-entity control and needs to be opened if it is to retain its ubiquity. The web runs on all kinds of devices and platforms and they all have special needs -- if you want to play here, you can't expect everyone to conform to your single distributed blob. Openness is the only way to keep prominence as a widespread web technology.
It's a little late, but I think Flash could still be saved if Adobe opened it now. Otherwise, it will shortly be killed by open alternatives like HTML5.
If they don't do this, Flash is dead, which is stupid imo because they could still make a lot of money off of the IDE.