The thing is, Flash is still in use, just not on the web and it's not called Flash. It's called Adobe Animate. Adobe Animate, formerly Flash, is still in use for animating shows on television. Open sourcing Flash would mean creating a potential competitor with Animate, which Adobe surely doesn't want. The only reason Adobe would have to open source Flash is "out of the kindness of their hearts" which is not enough. This is nothing but a needlessly risky business venture to them.

They wouldn’t need to open source the editor, only the player, so competing with Animate shouldn’t be an issue. But yes, there’s not much incentive to open source anything.

Or they could get some of their lawyers to get together a libre but not commercially available license.