I don't think Jobs's letter killed Flash. But I don't think Adobe did either.
no, the internet rallied around Flash like a mafia hitsquad around a mole and slowly beat it to death. NodeJS, html5, and webm video all colluded to deliver the killing stroke to Flash. Adobe, in turn, largely did what they do best and ignored the programs compatibility issues in Linux, stability issues in mac and windows, and rampant security issues across the board. It should serve as a stern reminder of what could happen to Photoshop and AfterEffects if Adobe doesnt start paying more attention and start fixing real bugs.