Scalability predictions from a software developer need to be taken with a grain of salt. There is no universal law determining it. The prediction is sound up until it runs into somebody else's fuckup, at which point it becomes worthless.

The idea that most software engineers can make any real guarantees about the correctness of their programs is laughable. At least once a month we hear about a massive new vulnerability that proves that that just isn't true. To not accept it is hubris.

It's not about calculating performance. It's about proving that something fundamentally fits in with everything we know about the world. Software developers that approach their work with this mindset are exceedingly rare, and their work is largely academic.