I didn't leave _that_ long ago, and don't want to look up my contract/NDA to see when it expires, so I don't want to go into too much detail.
Flash cost Adobe a lot to develop (it was a big team). At some point, I speculate[+] that Adobe simply decided they no longer need the runtime, since the browsers have/will soon have all the capabilities that they need, natively. So why waste the money?
[+] I don't know for sure (e.g. I didn't participate directly in the decision); but still, I believe my speculation is much more informed that the typical net user who just noticed the coincidence and wanted to believe the narrative.
"Adobe simply decided they no longer need the runtime, since the browsers have/will soon have all the capabilities that they need, natively."
I think that ignores a very very significant reason for the browsers getting to that point of being able to supplant Flash being that is was terrible and security nightmare and Apple had shifted mindshare against it and shown it wasn't needed in a mainstream platform. It wasn't just Adobe graciously terminating a successful platform that gave them a lot of influence.
Never claimed that it was without problems. But look at the timeline: Steve Jobs letter was April 29, 2010. About a year later Adobe stopped development of Flash on mobile (that's when they really killed it, the next 10 years were necessary to implement the decision basically). You really think that letter shifted mindshare in 1 year? Trust me, the opposite was true, there were some really big players that wanted to announce flash support on mobile and were caught completely off guard.
Using flash cc several years before EOL, it just wasn't a particularly consistent environment to develop in regardless of target device, and if you wanted to make ipad apps you ran into lots of limitations on what you could actually do with a release build, not sure if flash ever got the opportunity to use runtime interpreted actionscript 3 on release builds for iOS... that would have been nice to have.
