> Flash should have transitioned into an authoring tool for SVG + CSS + JS

Didn’t it? IIRC, Adobe had such a tool at some moment, and part of it seems to (somewhat) live on [1]. https://en.wikipedia.org/wiki/Apache_Flex:

“Apache Flex, formerly Adobe Flex, is a software development kit (SDK) for the development and deployment of cross-platform rich web applications based on the Adobe Flash platform. […] Adobe donated Flex to the Apache Software Foundation

[…]

In 2014, the Apache Software Foundation started a new project called FlexJS to cross-compile ActionScript 3 to JavaScript to enable it to run on browsers that do not support Adobe Flash Player and on devices that do not support the Adobe AIR runtime. In 2017, FlexJS was renamed to Apache Royale. The Apache Software Foundation describes the current iteration of Apache Royale as an open-source frontend technology that allows a developer to code in ActionScript 3 and MXML and target web, mobile devices and desktop devices on Apache Cordova all at once”

[1] I may be wrong though. It’s not easy figuring out what Flash code ended up in which of Adobe’s Flash-like products over time.