If I were YouTube, I'd archive or discontinue less used videos before I'd discontinue those with recent use. It would only make sense. So after some mean lifetime, any video that wasn't getting used might go in an "archive" with retrieval only if it got enough "votes" to bring it back. If Uncle Dave's stuff is being used with a book, it may be given a longer span of use. I know of no maximum for YouTube videos now, and if the free email services are any indication, hard drive storage won't be the issue. Server bandwidth is more likely to be the limiting factor in terms of investment. (As for the book. I'm glad to hear it's innovative. I got the feeling "Uncle Dave" was one clever fellow!)
There is a program out there called Tubesock that allows you to download and save videos from Youtube.
I do get a lot of my instruction from YouTube. Iespecially like http://www.youtube.com/user/tafkam1 I find his teaching and is playing to be wonderful and very understandable. I got alittle distracted in the beginning but now I just watch the ukulele ;-)