The main reason that big companies stop using Rails is because they have A LOT of traffic, and is because other low-level languages are faster to serve requests than Rails.
However, you don't choose Rails because is faster than other frameworks. You choose it because it allows you to code things fast in a very elegant way (the keynote of DHH for the RailsConf of this year was related to this).
Eventually, if at some point in time you need to handle a lot of users (so many that Rails starts slow down), you will probably have money to handle that (i.e. buy/improve hardware, hire more programmers to improve your app, etc).
And finally, if you're worried about job opportunities, there are enough out there (maybe not much as javascript, java, or other technologies, but there are enough jobs :-))