No no no no NO. It's time to get rid of Flash. Open-sourcing will make it live forever. Flash has very little to offer that is not at this point duplicated (or improved upon) by others. It's also woefully insecure. "Many eyes make all bugs shallow" will only work for the most trivial bugs in the most common code paths. Plenty of vulnerabilities will remain. In open source, they'll be even easier for attackers to find…
Preservation and reduction of bit rot is a great reason to open source Flash or ensure there is always a definitive version available, even if all the browsers should ban it because it is a messy, unperformant security liability.
Maintaining the build systems, compatibility layers and so forth won't be easy (Adobe often couldn't manage it), and bug fixing in the code is a risky prospect for preservation as it could render older content unplayable or differently playable.
The swf file format is almost open - it would be nice if Adobe updated the published specification to the latest format. Again that would go some way towards ensuring the content remained available in some way.
Edit: genuinely interested in the downvotes here - am I missing something fundamental about content preservation? It seems logical to me that the file spec being available + original players is far more valuable than the source, as the source will then bitrot without continuous maintenance. In comparison the players will always run in the VMs.