Earlier quoted context omitted.
The Flash runtime was not power hungry. It was the terrible code that was written that ran on it that was power hungry. As HTML5 was emerging someone posted a HTML 5 demo on hacker news. It put 1 core of my machine to 80%. I wrote an equivalent version in Flash and it used 5%. All the people writing terrible code in Flash simply started writing terrible code in Java script instead.
I never liked Flash, but the browsers were very far behind initially. Animation was slow and stuttering. At some point the browser vendors started addressing that, and things got better. But I think the other part of Flash is the tools used to create it - I never understood why they didn't port those to HTML5 and worked with the browser vendors to get kinks ironed out, instead of doubling down on - what was the name…
In late 2000s they owned the Web App space ( what was called Rich Internet Applications in their speak). Flex (their business GUI components library and markup language) was absolutely dominating corporate Web app development.
And they just abandoned it.
In an alternative reality we are all building HTML5 Web apps with Flex Builder right now.