Swift on Rails, who's first :) ?
I suspect Heroku are working on preparing for an out of the box build pack, but I guess they don't have access yet since it's out late 2015. At least I hope so, looking for a better Ruby replacement but I still like Rails.
I never considered this! But it's fun to think about!
I'm a bit naive about the environments languages have to run in... but if we have a web app framework in Swift, Servers in swift (as mentioned in comments above), and iOS in swift, doesn't that make Swift a JS replacement too?
JavaScript's domain is mostly client side web browser scripting, and no language is close to throwing it off its throne. So no, JavaScript will always have its place.