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.