i don't even necessarily think that it won't be around for a long time.
look at java. it'll be here for a long time to come. and it was very hot for a few years. but now its the cool, safe, standard language CS people pick up. and ruby, for rails, is the new hot language (which was virtually unknown outside of japan for many years until rails hit).
how long till rails cools down and something else picks up steam? how long until the answer to the question "i'm new, what language should i learn?" is something other than "ruby"?
on the flip side of that coin, COBOL programmers get paid a pretty penny nowadays because there are so few of them, but so many COBOL programs are still alive. how long until java programmers become the COBOL programmers of today?
whats "worth" learning depends on what you want to do and whats happening in the industry currently. imo.