Does Google foresee a future entirely built in Go, the same way NeXT expected the world to evolve towards Objective-C?
If there are equivalent ways of designing for concurrency in Java or C# (or those ways are added to the language in the future), then you may be able to restructure existing Java and C# code bases to take advantage of this approach.
If you have 20 million lines of Cobol transaction processing code and there's no practical way of migrating your code base to use this methodology, then your 20 million lines of Cobol will keep running the same way they always have.
Unless I'm missing a point you're trying to make (certainly possible) there's no reason to think that this is an all or nothing approach.
The upcoming 4.5 release fixes some of the ugly.
Eventually, sure, why not?
Still you don't need your whole OS to be in pure Go to take advantage of Go's features, most Go systems are purely in Go as the stdlib is quite complete, but even then there is no issue linking with (most) existing C code if you need to (C code that does its own threading stuff obviously is more problematic).
It looks like we are headed toward a future with many more languages, not fewer.
It may be a long time but there will someday be a world not built on C or C++. Probably within my lifetime. (Probably.)