> ... and then it was all killed because Apple did not dare to open source it, or put serious efforts on their own into improving the technical base of the flash player (that had aquired lots of technical dept).

IIRC, they couldn't open source Flash due to its use of a number of 3rd party C/C++ libraries that were proprietary.

Adobe's license with these 3rd parties permitted binary-only distribution so it would have meant renegotiating a fresh license (and paying out $$$) for an EOL codebase that had enormous technical debt, as you also acknowledge in your last sentence.