In Adobe's defense, they kinda did try that: AS3 was supposed to become JS2/ES4 and they also released a bunch of somewhat-useful documentation on the SWF file format. They even released AVM2 as free software (called "Tamarin") with the idea that Firefox would replace SpiderMonkey with it. The web community stuck it's nose up at ES4 and the SWF documentation was kind of half-assed.

Adobe's idea at the time of the HTML5 transition was that they were going to completely replace Flash with the Edge Suite, while Flash Player became an expensive-to-license compilation target for Unity. Hence, the whole "premium licensing" debacle that smacked of "we don't know who our customers are, but some of them aren't using Creative Suite, so let's try and charge them lots of money for things the web can already do".