Why in the world is github still on rails?
And perhaps they have little to gain and possibly much to lose if they ditch it?
You didn't say much in your question, so I don't know if you feel they ought to rewrite with a popular SPA framework or use something like Elixir Phoenix, but if their Rails-based solution handily serves 30 million users, why do you feel so strongly they should move to something else?
If Github wanted to integrate a lot of real-time features, then Elixir + Phoenix can't be beat. Depending on what they replace, a 10x in performance and a fraction of the servers needed is a nice win.