Languages seem to find a "killer app" much like most technologies and then tend to be used primarily for that purpose. Could Go do all of those things? Sure, with enough time, resources, and libraries, it could. But Google has a problem now (in scale) that Go excels at and can provide value now, so they use it for that.
That said, I would guess that most of Google's code is still C++, Python, and Java.
There's two senses of "scale:" large software, or software running with large data sets. The article uses the first notion: "at scale, for large programs with large numbers of dependencies, with large teams of programmers working on them."
So Go was designed for large software systems, and maybe it's really good at those things. Or maybe not, because nobody's written one yet. The article says so: "We don't have enough experience yet, especially with big programs." So Go is unproven in this way.
Speculatively, my guess is that it won't be able to "at scale" its current state. For example, Go errors out with unused variables, imports, or indeed for all warnings. Imagine trying to make a piece of software like WebKit or LLVM compile on all of the platforms it supports, with all of the compilers they support, without introducing a single warning (or extraneous header or unused variable) on any of them. It would be a maintenance nightmare. You need some flexibility - demanding perfect hygiene for every cell on an NxM matrix of supported platforms is infeasible.
In the other sense of scale - large data sets - Go apparently works well enough to be involved in YouTube. That's cool and is a credit to it.
You're saying that if Go had the same design flaws as C/C++, then it wouldn't be possible to write LLVM in it.
But Go doesn't most of the design flaws you mentioned. For example, there are no multiple Go compilers wildly diverging in what they understand. That's unique to C++. In Go, like in Java or C#, a compiler either implements the whole spec or it cannot be called a compiler.
Go also doesn't have the portability problems that have accumulated in C/C++ over decades.
Go doesn't have macros (which enable the mess that C/C++ code finds itself in).
Go doesn't have manual memory management, pointer arithmetic, array access is always bounds checked etc. - it's radically safer language than C/C++.
I also fail to see the logic connection between unused variables or unused imports and any of that.
Or how fixing the error caused by unused variable is a "maintenance nightmare". Count the number if #ifdef statements in boost and then let's have a discussion about what is and isn't a maintenance nightmare.
> For example, Go errors out with unused variables, imports, or indeed for all warnings.
Go doesn't have warnings. The Go compiler errors out on compile errors. Unused variables and imports are two things that people typically assume should be warnings, and are surprised to find them as errors.
So I don't think your fears are well-founded here. Go is a much stricter and clearly defined language than C. There are far fewer edge cases and ambiguities. The spec is much easier to implement, and indeed there are two complete compiler implementations that agree with each other and the spec.
That WebKit or any other large C project could produce no warnings is, of course, hard to imagine. C compilers emit warnings like... well, I can't think of a clever simile. But the point is, it's totally normal for C programs to emit tons of warnings. But ANY Go program (valid or not!) can never produce any warnings. Ever.
That said, I would guess that most of Google's code is still C++, Python, and Java.