What usually happens is people decide to upgrade everything all at once, since they're investing the time anyway. So they upgrade to Ruby 2, Rails 4, JQuery 2,

What usually happens is people decide to upgrade everything all at once, since they're investing the time anyway. So they upgrade to Ruby 2, Rails 4, JQuery 2, etc. all at once. Since the big driver was to upgrade Rails initially, Rails becomes the focal point of all the once-deprecated-now-removed errors that occur. That being said, upgrading from Rails 2 -> 3 was pretty annoying.