Productivity and working knowledge of a tool do not count as rational merits? It takes years to really learn a language; a new framework and language per week is a stupid fad.
If I run a Java shop I want a clique of Java devs, not Golang or Rust devs.
There is some truth in this, but less than commonly believed.
A "multi cultural", Jack-of-all-trades kind of team, that have a broader knowledge of many tools and techniques but less in depth familiarity with any of them, is more likely to use less features from the languages and tools that they have picked for a few of their strong points only, and less likely to indulge themselves with a level of sophistication that benefits their ego (and job security) more than the project.
> If I run a Jave shop
So I know a Python shop must be a business that sells reptiles, but what does a Java shop sell?
If I run a Java shop I want a clique of Java devs, not Golang or Rust devs.