Even a cursory glance at the runtime performance difference between these two frameworks reveals that either this project won't scale to the point that cloud costs are relevant or they have a dubious prioritization of DX over deployment economy. We are talking orders of magnitude fewer RPS for Rails.
I don’t understand your integration of performance and cloud costs here. “Deployment economy” is also new. Rails has a very strong track record of matching internet scale. Cloud is highly optimized for traditional server applications. From my experience with Next.js - this is the opposite. A lot of deployment components that don’t naturally fit in, and engineering required to optimize costs.
Quite simply: at certain threshold counts of users you will be forced to add many more cloud instances/pods running Rails than you would need running node.js (or Java or go or many others). But it doesn't stop at instances because this will also require more persistent disk / object storage, more logs, more alerts, more notifications from the cloud provider that instance xyz needs to be restarted (due to a firmware upgrade or whatever), etc. etc. All of these have human management overhead costs and most of them increase monthly financial costs.