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".
In my opinion it all was just too little too late. They should been investing heavily into open source as soon as Google started their crusade on web with Chrome. By the time WebGL and other APIs became mature everything was over for Flash.
Adobes evangelists really started running in circles after that.
And adobe itself made one stupid management decision after the other. And after they realised they will not get back on the ipads, they abondoned flash.
And this, after AIR ((flash for desktop) and Flex (UI Libary) were on its way to become established. Flex was really powerful. I would have loved to still use it, with a open-source flashplayer as its base. But adobe could not get themself waging to do that, so they just let it die.
There would be causalities - accessibility, responsive design, ad block (cosmetic and network), personalization (stylus). Yet with still unmatched creativity, maybe with stronger case against native applications (like today fuchsia). World would be different and we would never know exactly how.
They was so over controlling so there was whole game development industry where literally every company used Flash for UI for decade, but with 3rd-party proprietary runtime simply because Adobe ignored that market completely.
If they only moved Flash into open source when it was on peak of it's popularity they would probably still sell their proprietary tools to so much larger audience.