The Rails master branch was moved to 4.0 two months ago, almost 6 years to the date of the announcement of Rails 1.0, and the primary cited reason for doing so is to get everyone using Rails on Ruby 1.9.
[I understand the lack of migration all too well. Some codebases require a tremendous amount of work to move forward, and may not be worth the investment from a business point of view. But at some point, that app either needs to be sunset or worked on. Now’s the time to decide which category it fits into.]
Source: Union Station