Some technology stacks are clearly popular because of a 1-3 year long hype cycle. Rails/node/ember/mongo fall into this category I think, whereas others (python/java/C#/postgresql) build up more gradually but tend to stick around longer and don't rely upon a wave of hype.

I don't think node/ember will ever go away, but I'm sure that they'll lose their fashionability after a year or two much like rails has, and for that reason I'm pretty loath to chase them. I'd prefer to invest in the other type of technology instead of chasing faddish stuff.

These days the benefits of this model of development (one page js app) is also hyped enough to drown out the fairly significant drawbacks - javascript behaves unpredictably under different browsers, is weakly typed, and the whole model breaks certain abstractions. I'm betting that the hype will die down but the drawbacks aren't going away.

I also don't think that switching ecosystems is quite as easy as you make out. Each ecosystem has enormous tool chains and you quite often need years of experience to know what the best of 500 libraries/tools to use in any situation is.