As the communities grow, more people learn about mongodb's limitations and feel the need to switch. I like your "ruby" example because what mongodb faces is really similar: Easy to start with, hard to go... well... "web-scale" (by the way is this expresson becoming a word or has it already been?)

I usually do not link to my comments but I'll risk looking like a narcissist this time to give an example: https://news.ycombinator.com/item?id=6801970