I love it. Apologies if you didn't mean it this way, but given the usual context of this forum, the insinuation here is so clever: Imply that it takes significantly more "compute" costs for a Rails platform, as opposed to what it would take if it were running on something "more performant," say, Java, and the JS front end flavor of the week.

I would argue that the computing costs of any computing endeavor (short of AI, at this point) are dwarfed by the human costs, and my personal experience is that Ruby/Rails is at least a 10x headcount savings over the programmer sprawl required for Java/JS.

Java and JS sure are good for job security. I'll give them that. Throw in an unequivocal demand for Oracle, and Cisco networking gear, and you have the whole Fortune 500 world that got dumped on us from the people who couldn't manage the mainframes, either.

Ultimately the interesting thing to know is why they decided to stay on Rails instead of rewriting everything in a more performant language.

That they can solve it at all is critical information. One could have expected the stack to not be able to horizontally scale this far. You are right though, that there is also a question of a trade-off between hosting cost on one and and the developer hours, risk, opportunity cost and lower hosting cost that come with a rewrite on the other for which one would need to know hosting cost among other critical factors.