As someone who worked on a Merb app fulltime for about 4 years (before merb 1.0 through to well after rails 3 released) it's more accurate to say that Rails got rearchitected with some of the good ideas Merb had on the backend.

When the plan was originally announced, Yehuda blogged (http://yehudakatz.com/2008/12/23/rails-and-merb-merge/):

    In particular, we will do Merb releases with deprecation 
    notices and other transitional mechanisms to assist 
    developers in tracking down the changes that will come 
    between Merb 1.x and Rails 3. Expect a number of interim 
    releases that get incrementally closer to Rails 3, and 
    expect parts of Merb (most notably the helpers) to be 
    ported to run on Rails 3 in order to further reduce 
    friction.

This very specifically did not happen, and a lot of people are stuck on Merb or had a painful migration. Of note, Rails 3.0's first beta was released Feb 4 2010 (just over a year after the announcement), and 3.0.0 final was August 29. Merb had a 1.1 prerelease Feb 20, and the last version (1.1.3) July 10. Since then Merb has been dead.

Again Yehuda:

    You will not be left in the cold and we’re going to do 
    everything to make sure that your applications don’t get 
    stuck in the past.

Now I'm sure as part of the merb community I can take some small part in blame of this, but nobody, not merb developers, not rails developers, not merb users (to the best of my knowledge) wound up putting any serious stock into providing anything resembling a migration plan. Which is a shame.