1) I don't think space and bandwidth are an issue at all. Both of these are cheap and get cheaper as time goes by.

2) As long as people continue to visit the site, they will generate revenue.

3) If you aren't worried about this then I don't see why you would be worried about youtube going down the tubes.

Bottom line is that I think youtube will be around forever. I can't how it or why it would ever disappear. If there is even a hint that it would disappear, they are hundreds of other video sharing websites out there ready to take its place.