I was inside Adobe during the final death of Flash. Adobe as an organization is fully capable of maintaining a frighteningly complex product; Document Cloud is a testament to that. Steve Jobs and Apple's decision that iOS should be focused on mobile web (and later, the App Store) without Flash is what directly led to the end of the project.
I think that's a bit backwards. Flash provided a solidly mediocre experience. Non-native widgets (and consequently input issues and accessibility issues), high cpu usage and generally poor performance for things like video playback, poor security track record… on desktops. Avoid that on a more constrained platform was a no brainer.
Adobe's inability (or unwillingness?) to come up with a competitive product killed Flash. Jobs' unwillingness to hitch his wagon to Flash was a symptom not a cause.
And the relentlessly CVEs! It was banned on every corporate network at the time.
Once flash was used for ads, that didn't help either. It wasn't just "flash was slow", it was "webpages with a dozen yelling, flashing, dancing, cpu sucking flash ads" slow.
I wouldn't want that crap on my new shiny phone either.
