When the tide was turning against them with the rise of iOS, they should have full open sourced Flash player and made the authoring tools free to use, and encourage competing authoring tools.
Flash would likely have then been on a track towards full standardization and native support in browsers. Adobe's authoring tools would likely be best-of-breed and indispensable for high end web dev. All the security and performance and interoperability problems could have been solved over time. Flash is not all that different from an SVG to be honest.
One counterpoint to make: Flash sites were to my knowledge static layouts, the very opposite of responsive design. That is a big industry shift that Flash never made.
Instead Adobe threw in the towel and encouraged breaking a large portion of the old internet by deprecating it.
I think moves like these amount to hundred billion dollar drags on the economy. In a sense we're all a little bit poorer as a result. It's like digging a hole and filling it back up again. It's economic activity that benefits no-one.