Flash was the great motivator for browser vendors to improve [their] technology
Flash was the first technology to really compete with the open web. That competition (which went both ways, mind you; recall such late excesses as Flash's Text Layout Framework) ultimately made the web stronger and more versatile. It's hard to imagine what the world might look like today had that fire never been lit under the web standards movement.
At the same time it's not hard to imagine how much better the world would be today if Macromedia was not sold to Adobe, and Adobe was not scared into abandoning Flash by the concerted efforts of Apple, Microsoft and the web standards zealots.
Adobe wasn't scared in to abandoning Flash, they just stopped investing in in the early 2000s. Nothing prevented them from cleaning up the warts, adding an automatic updater a decade earlier, taking security seriously, investing in performance, etc.
Part of why the HTML5 group got so much traction was that everyone else realized that the web's rich application platform couldn't be a company which wasn't serious about maintaining it.
