Rewriting a huge software base from one language to any other is going to cost a lot more than throwing money at scaling infrastructure. If performance is actually a customer observable pain point, a company would be better off investing in performance tuning their own software as well as investing in people to work on Ruby itself.

Engineers love to pull out the "it doesn't scale" card with alternative language suggestions that usually align with whatever pet/fad language they've been playing with because its new and shiny (I accuse myself here as well). At some point a business is successful enough, like Stripe, where the risk of changing out an entire tech stack outweighs any perceived benefit.