Lack of iOS compatibility was the nail in the coffin. Our web agency already hated Flash because it lacked any SEO capability, and the editor was shit. When the iPhone dropped it we had already been building websites without it for a couple of years. It was just a nice accelerator and allowed us to deny clients who requested Flash much easier.
Fast forward a couple more years and security became the main issue. Many of our clients were infatuated with their Flash websites and didn't want to move on. I just took down a Flash website (replaced with HTML/CSS/JS) in... 2022!
Flash had been on shaky grounds for years. It was obvious that it had a limited lifespan, and that it eventually needed to die. I think even Adobe recognised this. But Apple accelerated it's death massively, and everyone seems to be very thankful for that.
There is a duolingo page that manages to kill my computer to death if I am not fast enough to close it. (Fortunately I only get it ~once a week.) Flash was easily blockable, but javascript, especially something that is compiled to javascript is much harder to catch.
Flash is dead. Autoplaying ads still exist.
For sites that used a Flash component (e.g. YouTube), the plug-in was fussy and flaky and would sometimes not work for any discernible reason, sometimes requiring an update to fix it. HTML5 replacements have been far less cantankerous.
Well...yeah. Flash sites usually make use of graphics, fonts and videos, all of which use bandwidth. They serve an entirely different purpose than regular HTML/CSS/JS pages.
Flash loaded everything at once, so that the experience was instantaneous and fluid, once it loaded. It would continue to work if you disconnected. There wouldn't be any buffering or lag due to network issues.
It was definitely what drew the first blood.
But in fairness, a lot of people were crying for blood, flash had issues all over, and it was a complete security disaster. Apple's decision basically gave everyone the OK to dump on it.
It also happened at a time where the idea of open web was ascendent, MSIE was being actively fought against, and the people involved were out to excise proprietary platforms.
Also all mobile platform flash implementations had been absolutely awful, barely usable battery killer even ignoring that the UI affordances were just not adapted for mobile touchscreen devices.