"For millions of users the CPU cost difference probably justifies the rewrite cost." This is only true if you have expensive computations done in Ruby or Python or similar, which is very rarely the case.
Not true, Ruby and Python are absurdly slow at even trivial tasks. Moving stuff around in memory, which is most of what a webapp is, is expensive. Lots of branches is gonna be really expensive too.
I've got more than 15 years of Rails production experience, including a lot of performance optimisation, and in my experience the Ruby code is very rarely the bottleneck. And in those cases, you can almost always find some solution.
