YouTube isn't MOVING away from Flash .. they are testing video in HTML5 because it is a new technology that is being considered for a future HTML Standard.. they would look bad if they DIDN'T test it.
YouTube will not move from Flash... by doing so.. they would block all the millions of users who use IE ... which accounts for more than half of web users.. they would also cause inconstancy in browser version.
For YouTube and any other company.. Flash runs on all browsers.. all OS's .. and that's what they care about.
Sure they will. If people are using IE they will see the nice little reminder/question at the top of the page that says something like, "Can't view these videos? Get a real browser and download Google Chrome now!" Google will then have someone else using their browser (which is worlds better than IE).