I think it's less about becoming mastery of tools. I don't think that is what's being advocated, and at least not my interpretation.
In your case, assuming that your field is "web development", this means getting deep mastery in web development. You know which language and framework you want to use in the backend for low traffic websites, and you know what you want to use for high concurrency websites. You know very well the relative strengths of PHP / Laravel and (say) Elixir / Phoenix.
If more on the frontend side of things, you feel comfortable enough to switch between various frameworks with a short learning curve, and you understand _why_ new frameworks and new ideas around how to build webapps are appearing. You also know when to use vanilla-js and when to use react. When to use ES6, TypeScript and Clojurescript
My interpretation of what might be counter-productive to mastery, is to get into webdev today, data science two years from now, distributed systems another 2 years, and then after that desktop applications etc whatever, based on the current trend. A careful judgement call needs to be made carefully when switching domains.
Ultimately every domain is both very wide and very deep, and it's hard enough to get mastery in one, let alone jump into a new field that your peers have already been doing since the beginning of their careers.
In your case, assuming that your field is "web development", this means getting deep mastery in web development. You know which language and framework you want to use in the backend for low traffic websites, and you know what you want to use for high concurrency websites. You know very well the relative strengths of PHP / Laravel and (say) Elixir / Phoenix.
If more on the frontend side of things, you feel comfortable enough to switch between various frameworks with a short learning curve, and you understand _why_ new frameworks and new ideas around how to build webapps are appearing. You also know when to use vanilla-js and when to use react. When to use ES6, TypeScript and Clojurescript
My interpretation of what might be counter-productive to mastery, is to get into webdev today, data science two years from now, distributed systems another 2 years, and then after that desktop applications etc whatever, based on the current trend. A careful judgement call needs to be made carefully when switching domains.
Ultimately every domain is both very wide and very deep, and it's hard enough to get mastery in one, let alone jump into a new field that your peers have already been doing since the beginning of their careers.