I'm not even going to try to equate the pace of backend tech libraries with frontend, but you can't just learn one stack and be set on the backend. SQL->NoSQL, various queuing methodologies, diff. Automation platforms, different scripting languages rising and falling..,
You can be a Java developer on hibernate the same way you can be a frontend dev doing jquery for almost a decade but being on the cutting edge is the same deal on frontend as it is backend.
I totally agree with the point about always needing to learn on front-end because things are always changing...but having recently been exposed to the Java world there is a huge amount of stuff there too and if you're a server side dev then it's constantly changing e.g. you're org has a django app now but what about Go, Clojure, Scala, Haskell, etc, etc, etc. Different tech for different use cases but there are constantly evolving options out there.
Things are always changing but that's the beauty of it, isn't it?
You can be a Java developer on hibernate the same way you can be a frontend dev doing jquery for almost a decade but being on the cutting edge is the same deal on frontend as it is backend.