Adobe Flash has been widely adopted. Youtube uses it. Major TV stations use it. Ads on side bar, top bar use it. There isn't a 'place' you don't see Flash. It demands quite a bit of CPU power, Yet, the popularity will make switching a pain in the butt. Migration to HTML5 is inevitable. I believe Adobe will support their customers to do the migration and certainly hope all Flash-users have a seamless plan of migration.
Better late than never, way better than being saddled with an old legacy which was not optimized for the best use of available hardware.
Adobe has shown they don't fear making fundamental changes for such a big company.