Yes and no. Shumway is in a race to stay relevant as Flash fades from the web, but there will always be a long-tail of Flash content that would/will be lost when Adobe or browsers stop supporting the Flash plugin.
Even YouTube must continue to support Flash video for at least another year because many websites embedded Flash videos using YouTube's boilerplate embedding HTML with the Flash <object> tag. Google can't magically rewrite these third-party websites' HTML. :) Maybe YouTube can use some SWF that loads or redirects people to youtube.com? Or browsers could recognize YouTube's boilerplate embedding HTML and rewrite it with YouTube's HTML5 video.
This is indeed a major consideration: Flash will undoubtedly go away as a player, and if we don't want to lose all the legacy content with it as if it were stored on floppy disks, Shumway or something like it is the only solution.
Note that for a lot of content, it's not even required to use Shumway in the form of a browser extension or built-in system: it's perfectly possible to run SWFs with Shumway as a website-deployed player. E.g., the header on http://areweflashyet.com is deployed that way and works just fine in all modern browsers.
It's not gone from YT! It's certainly on its way out. But despite the announcement I still see Flash as default in Firefox and even in Chrome it sometimes falls back to Flash (happens to me when I start a video, put my laptop to sleep, and continue the video after resume).
It looks like the main change was enabling HTML5 for every video. I haven't had Flash enabled for a couple years – previously many videos (IIRC everything with ads) would display the missing plugin error page. Now it falls back to <video> but since they built it wrong, that only happens if Flash is completely unavailable — any sort of click-to-play will still result in using Flash.
Even YouTube must continue to support Flash video for at least another year because many websites embedded Flash videos using YouTube's boilerplate embedding HTML with the Flash <object> tag. Google can't magically rewrite these third-party websites' HTML. :) Maybe YouTube can use some SWF that loads or redirects people to youtube.com? Or browsers could recognize YouTube's boilerplate embedding HTML and rewrite it with YouTube's HTML5 video.