Why is it so that big social networks like facebook & youtube, when changing layouts on their user profiles?

That they give us a time of period when we can switch between the old %26amp; the new for a while, but then they give us a date when they are permanently switching us all over to the new version that in this case when it come to the youtube channel change. Why can't it be an option to keep the old version, but still change to the new, when we think it would be cool to have that. Why is this so?Why is it so that big social networks like facebook %26amp; youtube, when changing layouts on their user profiles?http://webupon.com/social-networks/would