I learned and fell in love with Rails because I was stuck working with Struts and I saw how much easier and better life could be. So, at least for me, the decision to use a different framework didn't follow what you describe. I'm pretty sure if I came across something that allowed me to solve my problem in an easier and better way than Rails would, I'd use that instead.

For example, if I were building a mobile API that I was fairly sure wouldn't require a web interface later on, I'd consider using something simpler and more performant than Rails. It's not a matter of being an early adopter vs. a conservative developer, it's a question of practicality. As it always should be.