Flash is mature. It’s supported by all major desktop browsers. It’s stable when used properly. If not, it crashes a lot, just like every other technology. It requires constant security updates, just like every other web technology. It doesn’t work well on most mobile devices, and for good reasons. It’s a content plugin, developed during the era of closed standards and unilateral corporate control of web technology. Websites that rely on Flash can present a unique (and often unparalleled) experience for the massive percentage of users on a desktop browser. Flash powers some amazing experiences that work consistently across all of the major browsers in a way that cannot be replicated without Flash technology.
See the argument here