To be clear, I don’t think Jobs’s letter killed Flash. But I don’t think Adobe did either. Eventually Adobe accepted Flash’s demise. What killed Flash was Apple’s decision not to support it on iOS, combined with iOS’s immense popularity and the lucrative demographics of iOS users. If Jobs had never published “Thoughts on Flash”, Flash would still be dead. The letter explained the decision, but the decision that mattered was never to support it on iOS in the first place.
I’d go further and say that Adobe’s purchase of Macromedia killed the old Adobe long before Flash itself was killed.
The Macromedia purchase was very controversial within Adobe at the time and talented people left. The decision to manhandle Flash into every Adobe product seriously eroded quality and squandered enormous resources that could have been used to create new open standards and great new products.
Adobe is a different company today because of Flash, and lesser for it.