To be clear, this is a significantly different situation than with iOS. With iOS, Flash not only isn’t preinstalled, it can’t be installed, period. With Mac OS X, users are free to download Flash Player from Adobe and install it themselves.
Apple spokesperson Bill Evans told me: “We’re happy to continue to support Flash on the Mac, and the best way for users to always have the most up to date and secure version is to download it directly from Adobe.”
As of right now, video sites will show the “missing plugin” warning in Safari. But most websites are already serving an iOS ready HTML5 content, so it’s a small switch to trigger it for desktop Safari users as well. This is not going to be a big issue. Heck, even Adobe just released a beta version of their HTML5 video player.