We Smalltalkers used to think the advantages of our language were so significant that it would take over the world. We had a huge productivity advantage over C coders. Then C++ came along and gave C coders just enough to let them improve their productivity and their ability to write larger more complex systems. It still wasn’t as good as Smalltalk, but it was better than C, and much more accessible to most programmers than Smalltalk.
C++ eventually sucked up all the oxygen and Smalltalk is now only a language for hobbyists and the occasional programming god. I think this is the most likely threat to the Rails surplus, that C# or Scala or something can do a good enough job that people can double their productivity with far less of a change in mindset or tools, and eventually no one will care about the ten times (or whatever) productivity of Rails.
There is an assumption in this that I don't agree with. The assumption is that rails/ruby needs to (or does) get most of it's growth from java/.net developers and completely ignores php.
Far more people use php and if rails/ruby is able to draw from that pool then it's in good shape, and rails/ruby is a great improvement over php for applications that aren't expected to be distributed to tons of people that want to install it on a $10/month host. Drawing from java/.net is gravy.