Maybe for some kind of multiplayer game which has massive bandwidth and CPU usage requirements and has to be supported by paper-thin advertising profit margins... When tiny performance improvements can mean the difference between profitable and unprofitable, then it might make sense to optimize but this... But for the vast majority of software, the cost of serializing JSON is negligible and not worth thinking about.

For example, I've seen a lot of companies obsess over minor stuff like shaving a few bucks off their JSON serialization or using a C binding of some library to squeeze every drop of efficiency out of those technologies... While at the same time letting their software maintenance costs blow out of control... Or paying astronomical cloud compute bills when they could have self-hosted for 1/20th of the price...

Also, the word scale is overused. What is discussed here is performance optimization, not scalability. Scalability doesn't care for fixed overhead costs. Scalability is about growth in costs as usage increases and there is no difference in scalability if you use ProtoBuf or JSON.

The expression that comes to mind is "Penny-wise, pound-foolish." This effect is absolutely out of control in this industry.