I'm not making a pure scalability argument. Your solution of scaling the rails tier still suffers from far higher latency thus making choice of language/VM rele

I'm not making a pure scalability argument. Your solution of scaling the rails tier still suffers from far higher latency thus making choice of language/VM relevant. Rails is very, very slow — even if you cache everything behind it. So yes, you can scale it up, but it is at least 10x more expensive to do so.