> If they did not work with exactly the tools and programming languages / libraries of the job that is described in the job offer at hand, their performance end

> If they did not work with exactly the tools and programming languages / libraries of the job that is described in the job offer at hand, their performance ends up more in the junior part of the scale and it will cost you dearly. This seems to be the exact opposite of what the OP said, is it not? Clarifying edit: It seems like you're saying that you need to have worked with a certain tech-stack for 10 years to be considered a senior developer, whereas the OP seems to be advocating that a "senior developer" will be able to handle any tech stack you throw at them in a reasonable amount of time. Their knowledge of the tech stack may impact their speed at first, but not the quality of the result.