YouTube couldn't have started when it did without flash, even if it's moved on to html5 now.
Also lots of indie game devs (e.g. team meat of super meat boy or binding of Isaac) got their start there, a lot of the early "upload your flash game here" might have paved the way for getting so many games on the app store model when it got going.
Had flash not existed, I suspect they would have used another video playback plugin, or rolled their own. I don't think YouTube's success was dependent on Flash. But the ubiquity of Flash certainly didn't hurt.
Imagine YouTube depending on QuickTime, Windows Media Player, RealPlayer, or Java Applets. Flash's user experience was just so much better than those options. I'd guess that YouTube couldn't have had that much success were there not such a ubiquitous (and downright good) plugin.
Also lots of indie game devs (e.g. team meat of super meat boy or binding of Isaac) got their start there, a lot of the early "upload your flash game here" might have paved the way for getting so many games on the app store model when it got going.