You’ll never be a master because you are missing a lot of solutions, paradigms and techniques that are common in other languages and that you will never learn staying forever in the same language.
Also I would like to see how you can become a master in web development using C++.
You will always be castrated compared to someone else that has a much vaster experience with multiple languages and domains.
I think I can support this notion with something a little more concrete. I program C++ (13 yrs) and I learned javascript and web dev 8 years ago. don't consider myself a master at either. But, having broad coverage over and aptitude in both of these realms has had a huge impact on my ability to make an impact in the real world, working at a robotics company that heavily leverages these technologies. There are large rifts between teams that form due to the relative scarcity of "cross-pollinated" contributors.
I think that Linus Torvalds is a master of operating system development, even though he's done all his work in C. And I'm pretty sure that he doesn't care about ever being a master in web development (just as most web developers are probably not interested in becoming kernel developers).
The "doing the same thing" part, you added it.
You can very much be a master if you "worked for the whole life in the same programming language" and doing multiple things with it.
But even limited to a narrow scope, you can also be a master C++ programmer doing only games programming or network services programming.
You don't have to do some of this and some of that to become a master programmer.
You might not have the domain expertise to write a database or do DSP, but a domain expertise is something different from C++ mastery.